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The value of an investment increases by x% during January

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Re: Nice question on percentage [#permalink]

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New post 29 Oct 2013, 10:28
AccipiterQ wrote:
Bunuel wrote:
prashantbacchewar wrote:
The value of an investment increases by x% during January and decreases by y% during February. If the value of the investment is the same at the end of February as at the beginning of January, what is y in terms of x ?
(200x)\(100 + 2x)
x(2 + x)\(1 + x)^2
2x\1 + 2x
x(200 + x)\10,000
100 –( 10,000 \ 100 + x)

I got this question on MGMAT exam and got it wrong during the test. Later during the review I was able to solve this question correctly. I felt I should post this.


Let the value of investment at the beginning of January be \(I\).
Then the value of the investment at the end of February would be \(I(1+\frac{x}{100})(1-\frac{y}{100})\).

We are told that these values are equal: \(I=I(1+\frac{x}{100})(1-\frac{y}{100})\) --> \(100^2=(100+x)(100-y)\)

Answer: E.

This question can also be done by picking some smart numbers for I, x and y (say I=10, x=100 and y=50).



how did you get rid of the 100s on the bottom of each fraction and end up with 10000 on the other side? If you took the (1+x/100) portion and multiplied teh whole equation by 100 to get rid of it wouldn't you end up with (100+X)*(100-y)=100, since when multiplying by 100 you get rid of the 100 beneath the Y as well?


\(1=(1+\frac{x}{100})(1-\frac{y}{100})\);

\(1=(\frac{100+x}{100})(\frac{100-y}{100})\);

\(100*100=(100+x)(100-y)\).

Hope it's clear.
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Re: The value of an investment increases by x% during January [#permalink]

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New post 16 Nov 2013, 12:41
So I have a couple of principles that I think we can apply here to solve the problem quickly. For successive % changes, we must realize the following:

    1) if x > y, the change is going to be positive, negative, or zero. For example, 100 is increased by 10% so we get 110. Then 110 is decreased by 2%, the result is ~108. So we have a net positive change. If 110 is decreased by ~9.09%, the result is 100. So we have a net change of 0. If 110 is decreased by ~9.99%, the result is 99, so the net change is negative.

    2) If x<y, the net change is always going to be negative. For example, 100 is increased by 10%, which gives us 110 and then 110 is decreased by 11% which gives us ~98.

    3) If x=y, the net change is always going to be negative. For example, 100 is increased by 10%, which gives us 110 and then 110 is decreased by 10% which gives us 99. This reminds me of my investment in Apple's stock. One day I'm up 3% and then the next day I'm down 3% and initially, I might think that I'm at a net change of 0, but I actually lost money :( . If my initial $100 goes up by 3%, I now have 103 (w00t w00t!), but if my $103 goes down by 3% the next day, I have less than my initial 100 because 3% of 103 is more than 3% of 100.

Now, for this problem, we are told that the net change is zero, so using the principles above, we know that x > y and more importantly, we get a net change of zero whenever we increase by 10% and decrease by a little more than 9%. So therefore, all we need to do is plug in 10 for x and the answer choice that gives us ~9 is correct.

I think if we remember these very easy principles, it will help us in the long run, because we are able to solve problems such as this one, in under 2 minutes.

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Re: The value of an investment increases by x% during January [#permalink]

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New post 03 Aug 2014, 18:34
Bunuel wrote:
prashantbacchewar wrote:
The value of an investment increases by x% during January and decreases by y% during February. If the value of the investment is the same at the end of February as at the beginning of January, what is y in terms of x ?
(200x)\(100 + 2x)
x(2 + x)\(1 + x)^2
2x\1 + 2x
x(200 + x)\10,000
100 –( 10,000 \ 100 + x)

I got this question on MGMAT exam and got it wrong during the test. Later during the review I was able to solve this question correctly. I felt I should post this.


Let the value of investment at the beginning of January be \(I\).
Then the value of the investment at the end of February would be \(I(1+\frac{x}{100})(1-\frac{y}{100})\).

We are told that these values are equal: \(I=I(1+\frac{x}{100})(1-\frac{y}{100})\) --> \(100^2=(100+x)(100-y)\) --> \(100^2=100^2-100y+100x-xy\) --> \(y=\frac{100x}{100+x}\) or which is the same \(y=100-\frac{10,000}{100+x}\).

Answer: E.

This question can also be done by picking some smart numbers for I, x and y (say I=10, x=100 and y=50).


Hi Bunuel,

How did you make the leap from ?

\(y=\frac{100x}{100+x}\) or which is the same \(y=100-\frac{10,000}{100+x}\).

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Re: The value of an investment increases by x% during January [#permalink]

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New post 03 Aug 2014, 23:16
russ9 wrote:
Bunuel wrote:
prashantbacchewar wrote:
The value of an investment increases by x% during January and decreases by y% during February. If the value of the investment is the same at the end of February as at the beginning of January, what is y in terms of x ?
(200x)\(100 + 2x)
x(2 + x)\(1 + x)^2
2x\1 + 2x
x(200 + x)\10,000
100 –( 10,000 \ 100 + x)

I got this question on MGMAT exam and got it wrong during the test. Later during the review I was able to solve this question correctly. I felt I should post this.


Let the value of investment at the beginning of January be \(I\).
Then the value of the investment at the end of February would be \(I(1+\frac{x}{100})(1-\frac{y}{100})\).

We are told that these values are equal: \(I=I(1+\frac{x}{100})(1-\frac{y}{100})\) --> \(100^2=(100+x)(100-y)\) --> \(100^2=100^2-100y+100x-xy\) --> \(y=\frac{100x}{100+x}\) or which is the same \(y=100-\frac{10,000}{100+x}\).

Answer: E.

This question can also be done by picking some smart numbers for I, x and y (say I=10, x=100 and y=50).


Hi Bunuel,

How did you make the leap from ?

\(y=\frac{100x}{100+x}\) or which is the same \(y=100-\frac{10,000}{100+x}\).



\(y = 100 - \frac{10000}{100+x}\)

\(y = \frac{100 * 100 + 100x - 100000}{100+x}\)

\(y = \frac{100x}{100+x}\)
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Re: The value of an investment increases by x% during January [#permalink]

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New post 03 Aug 2014, 23:37
russ9 wrote:
Bunuel wrote:
prashantbacchewar wrote:
The value of an investment increases by x% during January and decreases by y% during February. If the value of the investment is the same at the end of February as at the beginning of January, what is y in terms of x ?
(200x)\(100 + 2x)
x(2 + x)\(1 + x)^2
2x\1 + 2x
x(200 + x)\10,000
100 –( 10,000 \ 100 + x)

I got this question on MGMAT exam and got it wrong during the test. Later during the review I was able to solve this question correctly. I felt I should post this.


Let the value of investment at the beginning of January be \(I\).
Then the value of the investment at the end of February would be \(I(1+\frac{x}{100})(1-\frac{y}{100})\).

We are told that these values are equal: \(I=I(1+\frac{x}{100})(1-\frac{y}{100})\) --> \(100^2=(100+x)(100-y)\) --> \(100^2=100^2-100y+100x-xy\) --> \(y=\frac{100x}{100+x}\) or which is the same \(y=100-\frac{10,000}{100+x}\).

Answer: E.

This question can also be done by picking some smart numbers for I, x and y (say I=10, x=100 and y=50).


Hi Bunuel,

How did you make the leap from ?

\(y=\frac{100x}{100+x}\) or which is the same \(y=100-\frac{10,000}{100+x}\).



\(y=\frac{100x}{100+x}\)

Add / subtract 100 to RHS

\(y = 100 + \frac{100x}{100+x} - 100\)

\(y = 100 + \frac{100x - 10000 -100x}{100+x}\)

\(y = 100 - \frac{10000}{100+x}\)
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Re: The value of an investment increases by x% during January [#permalink]

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Re: The value of an investment increases by x% during January [#permalink]

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Re: The value of an investment increases by x% during January [#permalink]

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New post 21 Mar 2017, 15:43
I tried to pick smart numbers and then to test the answers

so: 100 * x->(100-y)*100*x
we need y

let's x=25
125->125*0,8
hence y=20%
if we test the cases, E works well

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Re: The value of an investment increases by x% during January [#permalink]

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New post 13 Jun 2017, 02:02
Bunuel

y= 100- 100(10000 + x) - I don't understand why I keep getting this: here are my steps

100-[y/100]= 1/ (100 + [x/100])
100-y= 100 /(10000 +x)
-y= 100 /(10000 +x) -100
y= -100/(10000 +x) + 100
y= 100 - 100/(10000 +x)

Please help

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Re: The value of an investment increases by x% during January [#permalink]

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New post 12 Jul 2017, 15:43
AccipiterQ wrote:
Bunuel wrote:
prashantbacchewar wrote:
The value of an investment increases by x% during January and decreases by y% during February. If the value of the investment is the same at the end of February as at the beginning of January, what is y in terms of x ?
(200x)\(100 + 2x)
x(2 + x)\(1 + x)^2
2x\1 + 2x
x(200 + x)\10,000
100 –( 10,000 \ 100 + x)

I got this question on MGMAT exam and got it wrong during the test. Later during the review I was able to solve this question correctly. I felt I should post this.


Let the value of investment at the beginning of January be \(I\).
Then the value of the investment at the end of February would be \(I(1+\frac{x}{100})(1-\frac{y}{100})\).

We are told that these values are equal: \(I=I(1+\frac{x}{100})(1-\frac{y}{100})\) --> \(100^2=(100+x)(100-y)\)

Answer: E.

This question can also be done by picking some smart numbers for I, x and y (say I=10, x=100 and y=50).



how did you get rid of the 100s on the bottom of each fraction and end up with 10000 on the other side? If you took the (1+x/100) portion and multiplied teh whole equation by 100 to get rid of it wouldn't you end up with (100+X)*(100-y)=100, since when multiplying by 100 you get rid of the 100 beneath the Y as well?


Can someone answer this for me? thanks
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Re: The value of an investment increases by x% during January [#permalink]

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New post 12 Jul 2017, 21:29
Smokeybear00 wrote:
AccipiterQ wrote:
Bunuel wrote:
Let the value of investment at the beginning of January be \(I\).
Then the value of the investment at the end of February would be \(I(1+\frac{x}{100})(1-\frac{y}{100})\).

We are told that these values are equal: \(I=I(1+\frac{x}{100})(1-\frac{y}{100})\) --> \(100^2=(100+x)(100-y)\)

Answer: E.

This question can also be done by picking some smart numbers for I, x and y (say I=10, x=100 and y=50).



how did you get rid of the 100s on the bottom of each fraction and end up with 10000 on the other side? If you took the (1+x/100) portion and multiplied teh whole equation by 100 to get rid of it wouldn't you end up with (100+X)*(100-y)=100, since when multiplying by 100 you get rid of the 100 beneath the Y as well?


Can someone answer this for me? thanks


Please read the whole thread before posting a question: https://gmatclub.com/forum/the-value-of ... l#p1285304
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Re: The value of an investment increases by x% during January [#permalink]

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New post 13 Jul 2017, 02:47
prashantbacchewar wrote:
The value of an investment increases by x% during January and decreases by y% during February. If the value of the investment is the same at the end of February as at the beginning of January, what is y in terms of x ?

A. (200x)\(100 + 2x)
B. x(2 + x)\(1 + x)^2
C. 2x\1 + 2x
D. x(200 + x)\10,000
E. 100 –( 10,000 \ 100 + x)


Let initial investment at the beginning of January be I .
So after January total value of investment = (1+x/100)I
After February total value of investment = (1+x/100)(1-y/100) I

Now initial investment at the beginning of January = value of investment at the end of February
I = (1+x/100)(1-y/100) I
1= (100+x)/100 * (100-y )/100
10000= (100+x)(100-y )
100 - y = 10000/(100+x)
y = 100 - 10000/(100+x)

Answer E.
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The value of an investment increases by x% during January [#permalink]

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New post 14 Jul 2017, 07:11
Quote:


how did you get rid of the 100s on the bottom of each fraction and end up with 10000 on the other side? If you took the (1+x/100) portion and multiplied teh whole equation by 100 to get rid of it wouldn't you end up with (100+X)*(100-y)=100, since when multiplying by 100 you get rid of the 100 beneath the Y as well?


Quote:
Can someone answer this for me? thanks


Quote:
Please read the whole thread before posting a question: https://gmatclub.com/forum/the-value-of ... l#p1285304


I did and no one has answered his question.
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Re: The value of an investment increases by x% during January [#permalink]

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New post 14 Jul 2017, 07:16
Smokeybear00 wrote:
Quote:


how did you get rid of the 100s on the bottom of each fraction and end up with 10000 on the other side? If you took the (1+x/100) portion and multiplied teh whole equation by 100 to get rid of it wouldn't you end up with (100+X)*(100-y)=100, since when multiplying by 100 you get rid of the 100 beneath the Y as well?


Quote:
Can someone answer this for me? thanks


Quote:
Please read the whole thread before posting a question: https://gmatclub.com/forum/the-value-of ... l#p1285304


I did and no one has answered his question.


It was answered, hence the link. Here it is again: https://gmatclub.com/forum/the-value-of ... l#p1285304
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