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In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?

(1) e = 0.5 whenever i = 60
(2) p = 2.0 whenever i = 50

Originally posted by Superhuman on 07 May 2008, 00:49.
Last edited by alexsr on 01 Jul 2017, 10:41, edited 7 times in total.
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gettinit wrote:
Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.

$$a$$ is directly proportional to $$b$$ means that as the absolute value of $$b$$ gets bigger, the absolute value of $$a$$ gets bigger too, so there is some non-zero constant $$x$$ such that $$a=xb$$;

So if $$a$$ is directly proportional to $$b$$ ($$a=xb$$), then vise-versa is also correct: $$b$$ is directly proportional to $$a$$ ($$b=\frac{1}{x}*a$$ as the absolute value of $$a$$ gets bigger, the absolute value of $$b$$ gets bigger too).

$$a$$ is inversely proportional to $$b$$ means that as the absolute value of $$b$$ gets bigger, the absolute value of $$a$$ gets smaller, so there is some non-zero constant constant $$y$$ such that $$a=\frac{y}{b}$$.

So if $$a$$ is inversely proportional to $$b$$ ($$a=\frac{y}{b}$$), then vise-versa is also correct: $$b$$ is inversely proportional to $$a$$ ($$b=\frac{y}{a}$$ as the absolute value of $$a$$ gets bigger, the absolute value of $$b$$ gets smaller).

As for the question:
In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?

Given: $$p=ex$$ and $$e=iy$$ (for some constants $$x$$ and $$y$$), so $$p=ixy$$. Question: $$p=70xy=?$$ So, basically we should find the value of $$xy$$.

(1) e = 0.5 whenever i = 60 --> as $$e=iy$$ then $$0.5=60y$$ --> we can find the value of $$y$$, but still not sufficient.
(2) p = 2.0 whenever i = 50 --> as $$p=ixy$$ then $$2=50xy$$ --> we can find the value of $$xy$$. Sufficient.

Hope it's clear.
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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1
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we need P when i is some value...

we know p is dependent on e and e is dependent on i

In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?

1) e = 0.5 whenever i = 60 -> does not give the value or relation between e and P thus insufficient
2) p = 2.0 whenever i = 50 -> gives the relation between p and i thus we can find p when i=70

thus B
##### General Discussion
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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gettinit wrote:
Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.

production index p is directly proportional to efficiency index e,
implies p = ke (k is the constant of proportionality)

e is in turn directly proportional to investment i

implies e = mi (m is the constant of proportionality. Note here that I haven't taken the constant of proportionality as k here since the constant above and this constant could be different)

Then, p = kmi (km is the constant of proportionality here. It doesn't matter that we depict it using two variables. It is still just a number)

e.g. if p = 2e and e = 3i
p = 6i will be the relation. 6 being the constant of proportionality.

So if you have i and need p, you either need this constant directly (as you can find from statement 2) or you need both k and m (statement 1 only gives you m).
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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francoimps wrote:
VeritasPrepKarishma wrote:
gettinit wrote:
Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.

production index p is directly proportional to efficiency index e,
implies p = ke (k is the constant of proportionality)

e is in turn directly proportional to investment i

implies e = mi (m is the constant of proportionality. Note here that I haven't taken the constant of proportionality as k here since the constant above and this constant could be different)

Then, p = kmi (km is the constant of proportionality here. It doesn't matter that we depict it using two variables. It is still just a number)

e.g. if p = 2e and e = 3i
p = 6i will be the relation. 6 being the constant of proportionality.

So if you have i and need p, you either need this constant directly (as you can find from statement 2) or you need both k and m (statement 1 only gives you m).

Hi Karishma,

If I were to follow the solution for your post on joint variations in this blog http://www.veritasprep.com/blog/2013/02 ... mment-5837,

I would arrive with the solution:
p/e = k and e/i = k
hence, pi/e = k is the joint variation.

Why does this problem differ?

Thanks

Joint variation gives you the relation between 2 quantities keeping the third (or more) constant.
p will vary inversely with i if and only if e is kept constant.

Think of it this way, if p increases, e increases. But we need to keep e constant, we will have to decrease i to decrease e back to original value. So an increase in p leads to a decrease in i to keep e constant.
But if we don't have to keep e constant, an increase in p will lead to an increase in e which will increase i.

Here, we are not given that e needs to be kept constant. So we will not use the joint variation approach.
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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francoimps wrote:
VeritasPrepKarishma wrote:

Joint variation gives you the relation between 2 quantities keeping the third (or more) constant.
p will vary inversely with i if and only if e is kept constant.

Think of it this way, if p increases, e increases. But we need to keep e constant, we will have to decrease i to decrease e back to original value. So an increase in p leads to a decrease in i to keep e constant.
But if we don't have to keep e constant, an increase in p will lead to an increase in e which will increase i.

Here, we are not given that e needs to be kept constant. So we will not use the joint variation approach.

Hi Karishma,

How will I know whether the question asks that a certain variable needs to be kept constant?

The question above, "In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?" seems similar to the question on your blog post, "x varies directly with y and y varies inversely with z."

What should I explicitly look for to determine whether the issue is joint variation or not?

It will be told that the third variable has to be kept constant.

Note how the independent question is framed in my post:
The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical M present and inversely proportional to the concentration of chemical N present. If the concentration of chemical N is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical M required to keep the reaction rate unchanged?

You need relation between N and M when reaction rate is constant.
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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harishbiyani8888 wrote:
VeritasPrepKarishma wrote:
gettinit wrote:
Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.

production index p is directly proportional to efficiency index e,
implies p = ke (k is the constant of proportionality)

e is in turn directly proportional to investment i

implies e = mi (m is the constant of proportionality. Note here that I haven't taken the constant of proportionality as k here since the constant above and this constant could be different)

Then, p = kmi (km is the constant of proportionality here. It doesn't matter that we depict it using two variables. It is still just a number)

e.g. if p = 2e and e = 3i
p = 6i will be the relation. 6 being the constant of proportionality.

So if you have i and need p, you either need this constant directly (as you can find from statement 2) or you need both k and m (statement 1 only gives you m).

Hi Krishna/Bunuel,

Can you explain why the constant k could be different? I took constant k for both the proportionalities and marked answer D.

Let me ask you the flip question: why do you think both constants would have the same value?

"In a certain business, production index p is directly proportional to efficiency index e," - say, p = 2e so when e doubles, p becomes four times etc

"e is in turn directly proportional to investment i." - Now how does this imply that e = 2i? We could very well have e = 3i or e = i/2 etc

We are not given that whatever the relation is between p and e, the relation has to be the same between e and i too.
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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i say B as well, unless im missing something.

From stat 1, you know relationship btwn e and i, but you dont know what it is btwn p and e ... so insuff.

From stat 2, you are given the relationship btwn p and i, and from the stem you know what i is. so suff.
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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If P id directly proportional to E then what is the relation between them?

Is it only P = E * x

Or can it also be P = E*x + y.

In both the cases P is directly proportional to E. As in the question the author doesn't mention anything about the values of the variables when either of them is zero, it leads to a confusing situation.

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Re: In a certain business, production index p is directly proportional to  [#permalink]

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SrinathVangala wrote:
If P id directly proportional to E then what is the relation between them?

Is it only P = E * x

Or can it also be P = E*x + y.

In both the cases P is directly proportional to E. As in the question the author doesn't mention anything about the values of the variables when either of them is zero, it leads to a confusing situation.

It is P = E*k only.
It cannot be P = E*k + m

Directly proportional means that if one doubles, other doubles too. If one becomes half, other becomes half too.
It doesn't happen in case you add a constant.

P = 2E + 1
If E = 5, P = 11
If E = 10, P = 21
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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Bunuel wrote:
gettinit wrote:
Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.

$$a$$ is directly proportional to $$b$$ means that as the absolute value of $$b$$ gets bigger, the absolute value of $$a$$ gets bigger too, so there is some non-zero constant $$x$$ such that $$a=xb$$;

So if $$a$$ is directly proportional to $$b$$ ($$a=xb$$), then vise-versa is also correct: $$b$$ is directly proportional to $$a$$ ($$b=\frac{1}{x}*a$$ as the absolute value of $$a$$ gets bigger, the absolute value of $$b$$ gets bigger too).

$$a$$ is inversely proportional to $$b$$ means that as the absolute value of $$b$$ gets bigger, the absolute value of $$a$$ gets smaller, so there is some non-zero constant constant $$y$$ such that $$a=\frac{y}{b}$$.

So if $$a$$ is inversely proportional to $$b$$ ($$a=\frac{y}{b}$$), then vise-versa is also correct: $$b$$ is inversely proportional to $$a$$ ($$b=\frac{y}{a}$$ as the absolute value of $$a$$ gets bigger, the absolute value of $$b$$ gets smaller).

As for the question:
In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?

Given: $$p=ex$$ and $$e=iy$$ (for some constants $$x$$ and $$y$$), so $$p=ixy$$. Question: $$p=70xy=?$$ So, basically we should find the value of $$xy$$.

(1) e = 0.5 whenever i = 60 --> as $$e=iy$$ then $$0.5=60y$$ --> we can find the value of $$y$$, but still not sufficient.
(2) p = 2.0 whenever i = 50 --> as $$p=ixy$$ then $$2=50xy$$ --> we can find the value of $$xy$$. Sufficient.

Hope it's clear.

Hi Bunuel,

When you break it down like that, it makes complete sense but I made the following error. Can you please clarify why this isn't true?

$$\frac{p}{e}$$ = $$\frac{e}{i}$$

$$\frac{p}{.5}$$ = $$\frac{.5}{60}$$ and solve for p. If the ratios are proportional, shouldn't .5/60 give me a relationship for p/e since I already know E? This led me to choose "D" as the answer choice.

Thanks
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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russ9 wrote:
Bunuel wrote:
gettinit wrote:
Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.

$$a$$ is directly proportional to $$b$$ means that as the absolute value of $$b$$ gets bigger, the absolute value of $$a$$ gets bigger too, so there is some non-zero constant $$x$$ such that $$a=xb$$;

So if $$a$$ is directly proportional to $$b$$ ($$a=xb$$), then vise-versa is also correct: $$b$$ is directly proportional to $$a$$ ($$b=\frac{1}{x}*a$$ as the absolute value of $$a$$ gets bigger, the absolute value of $$b$$ gets bigger too).

$$a$$ is inversely proportional to $$b$$ means that as the absolute value of $$b$$ gets bigger, the absolute value of $$a$$ gets smaller, so there is some non-zero constant constant $$y$$ such that $$a=\frac{y}{b}$$.

So if $$a$$ is inversely proportional to $$b$$ ($$a=\frac{y}{b}$$), then vise-versa is also correct: $$b$$ is inversely proportional to $$a$$ ($$b=\frac{y}{a}$$ as the absolute value of $$a$$ gets bigger, the absolute value of $$b$$ gets smaller).

As for the question:
In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?

Given: $$p=ex$$ and $$e=iy$$ (for some constants $$x$$ and $$y$$), so $$p=ixy$$. Question: $$p=70xy=?$$ So, basically we should find the value of $$xy$$.

(1) e = 0.5 whenever i = 60 --> as $$e=iy$$ then $$0.5=60y$$ --> we can find the value of $$y$$, but still not sufficient.
(2) p = 2.0 whenever i = 50 --> as $$p=ixy$$ then $$2=50xy$$ --> we can find the value of $$xy$$. Sufficient.

Hope it's clear.

Hi Bunuel,

When you break it down like that, it makes complete sense but I made the following error. Can you please clarify why this isn't true?

$$\frac{p}{e}$$ = $$\frac{e}{i}$$

$$\frac{p}{.5}$$ = $$\frac{.5}{60}$$ and solve for p. If the ratios are proportional, shouldn't .5/60 give me a relationship for p/e since I already know E? This led me to choose "D" as the answer choice.

Thanks

Directly proportional means that as one amount increases, another amount increases at the same rate.

We are told that p is directly proportional to e and e is directly proportional to i. But it does NOT mean that the rate of increase, constant of proportionality, (x in my solution) for p and e is the same as the rate of increase, constant of proportionality, (y in my solution) for e and i.

Or simply put, we have that $$\frac{p}{e}=x$$ and $$\frac{e}{i}=y$$ but we cannot say whether x=y, so we cannot say whether $$\frac{p}{e}=\frac{e}{i}$$.

Hope it's clear.
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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VeritasPrepKarishma wrote:
gettinit wrote:
Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.

production index p is directly proportional to efficiency index e,
implies p = ke (k is the constant of proportionality)

e is in turn directly proportional to investment i

implies e = mi (m is the constant of proportionality. Note here that I haven't taken the constant of proportionality as k here since the constant above and this constant could be different)

Then, p = kmi (km is the constant of proportionality here. It doesn't matter that we depict it using two variables. It is still just a number)

e.g. if p = 2e and e = 3i
p = 6i will be the relation. 6 being the constant of proportionality.

So if you have i and need p, you either need this constant directly (as you can find from statement 2) or you need both k and m (statement 1 only gives you m).

Hi Karishma,

If I were to follow the solution for your post on joint variations in this blog http://www.veritasprep.com/blog/2013/02 ... mment-5837,

I would arrive with the solution:
p/e = k and e/i = k
hence, pi/e = k is the joint variation.

Why does this problem differ?

Thanks
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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VeritasPrepKarishma wrote:

Joint variation gives you the relation between 2 quantities keeping the third (or more) constant.
p will vary inversely with i if and only if e is kept constant.

Think of it this way, if p increases, e increases. But we need to keep e constant, we will have to decrease i to decrease e back to original value. So an increase in p leads to a decrease in i to keep e constant.
But if we don't have to keep e constant, an increase in p will lead to an increase in e which will increase i.

Here, we are not given that e needs to be kept constant. So we will not use the joint variation approach.

Hi Karishma,

How will I know whether the question asks that a certain variable needs to be kept constant?

The question above, "In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?" seems similar to the question on your blog post, "x varies directly with y and y varies inversely with z."

What should I explicitly look for to determine whether the issue is joint variation or not?
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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VeritasPrepKarishma wrote:
gettinit wrote:
Would p be directly proportional to i as well if e is proportional to p? I am thinking it should be, however the constant proportion will be different between p and e and e and i and thus entirely separate between p and i? thanks.

production index p is directly proportional to efficiency index e,
implies p = ke (k is the constant of proportionality)

e is in turn directly proportional to investment i

implies e = mi (m is the constant of proportionality. Note here that I haven't taken the constant of proportionality as k here since the constant above and this constant could be different)

Then, p = kmi (km is the constant of proportionality here. It doesn't matter that we depict it using two variables. It is still just a number)

e.g. if p = 2e and e = 3i
p = 6i will be the relation. 6 being the constant of proportionality.

So if you have i and need p, you either need this constant directly (as you can find from statement 2) or you need both k and m (statement 1 only gives you m).

Hi Krishna/Bunuel,

Can you explain why the constant k could be different? I took constant k for both the proportionalities and marked answer D.
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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Superhuman wrote:
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?

(1) e = 0.5 whenever i = 60
(2) p = 2.0 whenever i = 50

Given: p = c1*e - (i) where c1 is a constant
and e = c2*i - (ii) where c2 is a constant

Required: p = ? when i = 70
p = c1*e = c1*c2*i
p = c1*c2*70 - (iii)
Hence we need to find the value of c1 and c2

Statement 1: e=0.5 whenever i=60
From (ii),
0.5 = c2*60
Clearly we cannot solve for c1
INSUFFICIENT

Statement 2: p=2.0 whenever i=50
From (i),
2 = c1*50
and we know that
p = c1*70

Dividing both the equations,
2/p = 50/70
Hence we can solve for p
SUFFICIENT

Correct Option: B
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GMAT 1: 750 Q49 V43 Re: In a certain business, production index p is directly proportional to  [#permalink]

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Superhuman wrote:
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment i. What is p if i = 70?

(1) e = 0.5 whenever i = 60
(2) p = 2.0 whenever i = 50

Stimulus tell us that
pei
p=k1*e=k2*i {where k1 and k2 are proportionality constant for e and i respectively }
(1) e = 0.5 whenever i = 60
pk1*0.5k2*60
We do not know what the proportionally constant k1 and k2 are; there is no way to calculate them either.
INSUFFICIENT

(2) p = 2.0 whenever i = 50
2=k2*50
k2 =2/50
Since we know k2 we can figure out any relation between p and k
SUFFICIENT

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Re: In a certain business, production index p is directly proportional to  [#permalink]

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Hi Bunuel/Karishma

I solved this question and culminated in choice D as answer choice. I am posting my solution here, please help me with reason why i am wrong.

Statement 1 - e= 0.5 whenever i =60

per question stem we know that p is directly proportion to e and e is directly proportion to i , using this we can calculate P as 60*0.5 = 30 at e and i being 0.5 and 60 respectively; and to find the value of e (called e1) at i =70 . e = (0.5/60) *70 ---> 7/30 is the value of e (e1) at i = 70.

Now we can calculate p at i =70 using above relation = 30* (7/6) * (7/15)

where 7/6 is constant for i and 7/15 {7/(30*0.5)} is constant for e.

Using above i arrived at option D which is wrong. Please let me know me the pitfalls and where did i move away from the relevant concept.

Looking forward to hear from the Masters!!
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Re: In a certain business, production index p is directly proportional to  [#permalink]

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akbankit wrote:
Hi Bunuel/Karishma

I solved this question and culminated in choice D as answer choice. I am posting my solution here, please help me with reason why i am wrong.

Statement 1 - e= 0.5 whenever i =60

per question stem we know that p is directly proportion to e and e is directly proportion to i , using this we can calculate P as 60*0.5 = 30 at e and i being 0.5 and 60 respectively; and to find the value of e (called e1) at i =70 . e = (0.5/60) *70 ---> 7/30 is the value of e (e1) at i = 70.

Now we can calculate p at i =70 using above relation = 30* (7/6) * (7/15)

where 7/6 is constant for i and 7/15 {7/(30*0.5)} is constant for e.

Using above i arrived at option D which is wrong. Please let me know me the pitfalls and where did i move away from the relevant concept.

Looking forward to hear from the Masters!!

So, you are calculating p as p = ei, which is wrong. $$p=ex$$ and $$e=iy$$ (for some constants $$x$$ and $$y$$), so $$p=ixy$$.

Not sure what you are doing in the part in red.

Please check correct approaches above. They should help to get the flaws in your logic.

Theory:
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_________________ Re: In a certain business, production index p is directly proportional to   [#permalink] 28 Dec 2017, 20:51

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# In a certain business, production index p is directly proportional to  