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If the price of a commodity is directly proportional to m^3

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If the price of a commodity is directly proportional to m^3  [#permalink]

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New post Updated on: 07 Apr 2013, 22:20
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If the price of a commodity is directly proportional to m^3 and inversely proportional to q^2, which of the following values of m and q will result in the highest price for the commodity?

A. m=3, q=2
B. m=12, q=12
C. m=20, q=20
D. m=30, q=36
E. m=36, q=72

Originally posted by Dipankar6435 on 07 Apr 2013, 05:45.
Last edited by Bunuel on 07 Apr 2013, 22:20, edited 1 time in total.
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Re: Price of a commodity  [#permalink]

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New post Updated on: 07 Apr 2013, 06:00
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If the price of a commodity is directly proportional to m^3 and inversely proportional to q^2, which of the following values of m and q will result in the highest price for the commodity?
A. m=3, q=2
B. m=12, q=12
C. m=20, q=20
D. m=30, q=36
E. m=36, q=72

Let the price is P & constant is K
So P = \(\frac{K m^3}{q^2}\)

Now Just use the values in the options and put in the before mentioned equation.
A) P = \(\frac{K m^3}{q^2}\) = \(\frac{K 3^3}{2^2}\) = \(\frac{K 27}{4}\) = 6.75 K
B) P = \(\frac{K m^3}{q^2}\) = \(\frac{K 12^3}{12^2}\) = 12 K
C) P = \(\frac{K m^3}{q^2}\) = \(\frac{K 20^3}{20^2}\) = 20 K
D) P = \(\frac{K m^3}{q^2}\) = \(\frac{K 30^3}{36^2}\) = 125/6 = >20K
E) P = \(\frac{K m^3}{q^2}\) = \(\frac{K 36^3}{72^2}\) = 9 K

So the answer is D. This is probably 600-700 level question. :arrow:

Hope the explanation helps.
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Originally posted by emmak on 07 Apr 2013, 05:56.
Last edited by emmak on 07 Apr 2013, 06:00, edited 1 time in total.
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Re: Price of a commodity  [#permalink]

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New post 07 Apr 2013, 05:57
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Dipankar6435 wrote:
If the price of a commodity is directly proportional to m^3 and inversely proportional to q^2, which of the following values of m and q will result in the highest price for the commodity?
A. m=3, q=2
B. m=12, q=12
C. m=20, q=20
D. m=30, q=36
E. m=36, q=72


Function = \(\frac{m^3}{q^2}\)

A=\(\frac{3*3*3}{2*2}=\frac{27}{4}=7\) almost
B=\(\frac{12*12*12}{12*12}=12\)
C=\(\frac{20*20*20}{20*20}=20\)
D=\(\frac{30*30*30}{36*36}=125/6\)
E=\(\frac{36*36*36}{72*72}=9\)

It's down to C or D, because \(\frac{120}{6}=20\), \(\frac{125}{6}>20\)
D
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If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 07 Apr 2013, 22:22
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Dipankar6435 wrote:
If the price of a commodity is directly proportional to m^3 and inversely proportional to q^2, which of the following values of m and q will result in the highest price for the commodity?

A. m=3, q=2
B. m=12, q=12
C. m=20, q=20
D. m=30, q=36
E. m=36, q=72



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Re: If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 09 Apr 2013, 04:56
Let price of the commodity be P = \(k *\)\(m^3\) /\(Q^2\)

A. P = \(k * 27/4\) = 6.75K
B. P = \(K * 12^3/12^2\) = 12k
C. P = \(K * 20^3/20^2\) = 20k
D. P = \(K * 30^3/36^2\)= \(k*5*5*30/6*6\) = \(k * (125/6)\) = 21K (approx)
E. P = \(k * 36^3/72^2\) = \(K*(36/2*2)\) = 9k

Ans : Option D.
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Re: If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 10 Apr 2013, 18:09
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Could you please explain how you get \(\frac{m^3}{q^2}\) or \(\frac{Km^3}{q^2}\)? Where am I wrong expressing this thing firstly as \(p=m^3k\) and \(p=\frac{k}{q^2}\)?
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Re: If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 10 Apr 2013, 23:19
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obs23 wrote:
Could you please explain how you get \(\frac{m^3}{q^2}\) or \(\frac{Km^3}{q^2}\)? Where am I wrong expressing this thing firstly as \(p=m^3k\) and \(p=\frac{k}{q^2}\)?


"directly proportional to m^3 and inversely proportional to q^2" is what the text says. So if m increases the price increases, if q increases the price decreases. The right formula here is \(\frac{m^3}{q^2}\) ( or with k, it doesn`t change anything), if the numerator grows, the price does the same; and as in every fraction, if the denominator grows, the price drops.
Your formulas are right but the price depends on both m and q, you have to include them in one equation.

Let me know if it is clear
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Re: If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 11 Apr 2013, 03:51
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Zarrolou wrote:
obs23 wrote:
Could you please explain how you get \(\frac{m^3}{q^2}\) or \(\frac{Km^3}{q^2}\)? Where am I wrong expressing this thing firstly as \(p=m^3k\) and \(p=\frac{k}{q^2}\)?


"directly proportional to m^3 and inversely proportional to q^2" is what the text says. So if m increases the price increases, if q increases the price decreases. The right formula here is \(\frac{m^3}{q^2}\) ( or with k, it doesn`t change anything), if the numerator grows, the price does the same; and as in every fraction, if the denominator grows, the price drops.
Your formulas are right but the price depends on both m and q, you have to include them in one equation.

Let me know if it is clear


I see that makes sense. I guess I am making it more complicated than it really is. If you say that my formulas are correct, then \(\frac{m^3}{q^2}\) should somehow be extracted from \(p=m^3k\) and \(p=\frac{k}{q^2}\), no? Technically speaking, I thought there was a way to combine them into \(\frac{m^3}{q^2}\), just from pure algebraic manipulation (which I am very interested in here)... Or is it simply about common sense or the way you explained it?

P.S. Like your "kudos" tagline man! :)
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Re: If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 11 Apr 2013, 04:05
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obs23 wrote:
I see that makes sense. I guess I am making it more complicated than it really is. If you say that my formulas are correct, then \(\frac{m^3}{q^2}\) should somehow be extracted from \(p=m^3k\) and \(p=\frac{k}{q^2}\), no? I thought there was a way to combine them into \(\frac{m^3}{q^2}\), just from pure algebraic manipulation (which I am very interested in here)... Or is it simply about common sense or the way you explained it?

P.S. Like your tagline man! :)


To create the formula we should refer to the text:
"If the price of a commodity is directly proportional to m^3 [and at this point we write down Price= \(m^3\)] and inversely proportional to q^2 [and at this point we complete the foumula adding the Denominator so Price=\(\frac{m^3}{q^2}\)], which of the following values of m and q will result in the highest price for the commodity?"
\(\frac{m^3}{q^2}\) is not extracted from \(p=m^3k\) and \(p=\frac{k}{q^2}\), and you cannot obtain it from pure algebraic manipulation.
The idea behind your formulas is correct p=m^3 expresses the direct correlation between p and m; and also p=1/q^2 expresses the inverse correlation between p and q. But the text uses "and" so those ideas must be expressed in one formula, so to obtain this final formula you "complete" one with the other =>m^3/p^2
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Re: If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 11 Apr 2013, 04:30
Got it! Thanks much for help.
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Re: If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 11 Apr 2013, 07:22
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Dipankar6435 wrote:
If the price of a commodity is directly proportional to m^3 and inversely proportional to q^2, which of the following values of m and q will result in the highest price for the commodity?

A. m=3, q=2
B. m=12, q=12
C. m=20, q=20
D. m=30, q=36
E. m=36, q=72



For more on direct, inverse and joint variation, check out these posts:

http://www.veritasprep.com/blog/2013/01 ... -directly/
http://www.veritasprep.com/blog/2013/02 ... inversely/
http://www.veritasprep.com/blog/2013/02 ... g-jointly/
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Re: If the price of a commodity is directly proportional to m^3  [#permalink]

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New post 21 Oct 2017, 19:33
not sure why i dont understand this, but why does

30^3 / 36^2 = (30)(30)(30) / (36)(36) = (5)(5)(5) / 6

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If the price of a commodity is directly proportional to m^3  [#permalink]

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