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In a certain industry, production index x is directly propor [#permalink]
05 Feb 2013, 17:25

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95% (hard)

Question Stats:

35% (02:43) correct
65% (01:39) wrong based on 66 sessions

In a certain industry, production index x is directly proportional to the square of efficiency index y, and indirectly proportional to investment index z. If a business in this industry halves its investment index, which of the following is closest to the percent change in the business’s efficiency index required to keep the production index the same?

Re: In a certain industry, production index x is directly [#permalink]
05 Feb 2013, 20:50

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This post received KUDOS

Expert's post

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megafan wrote:

In a certain industry, production index x is directly proportional to the square of efficiency index y, and indirectly proportional to investment index z. If a business in this industry halves its investment index, which of the following is closest to the percent change in the business’s efficiency index required to keep the production index the same?

Next post will discuss a similar question using both. I have tried to show every step here to avoid confusion but on the blog, I am going to be more direct. By the way, the question is not very GMAT-like.

x = ky^2 (x is directly proportional to y^2. k is any constant) xz = k (I am assuming that indirectly is actually inversely proportional)

Both together, \frac{xz}{y^2} = k (Assume z to be constant, you get x is directly proportional to y^2. Assume y to be constant, you get x is inversely proportional to z)

\frac{x_1*z_1}{(y_1)^2} = \frac{x_2*z_2}{(y_2)^2} Given that z_2 = (\frac{1}{2})z_1 and that x_1should be equal to x_2. What is the relation between y_1 and y_2?

Re: In a certain industry, production index x is directly [#permalink]
05 Feb 2013, 20:58

Expert's post

megafan wrote:

In a certain industry, production index x is directly proportional to the square of efficiency index y, and indirectly proportional to investment index z. If a business in this industry halves its investment index, which of the following is closest to the percent change in the business’s efficiency index required to keep the production index the same?

(A) 100% increase

(B) 50% increase

(C) 30% increase

(D) 30% decrease

(E) 50% decrease

Source: Gmat Hacks 1800

The entire expression can be written in the form x=k*y^2/z. When z is halved, the expression becomes x=2k*y^2/z. In order to make this expression equal to its initial form, we need to divide this expression by 2. This can be done by reducingy to y/\sqrt{2}. Therefore percent change= y-y/1.414=>0.414y/1.414. This is almost equal to 30%. Hope this helps. _________________

Re: In a certain industry, production index x is directly propor [#permalink]
05 Feb 2013, 23:34

Expert's post

megafan wrote:

In a certain industry, production index x is directly proportional to the square of efficiency index y, and indirectly proportional to investment index z. If a business in this industry halves its investment index, which of the following is closest to the percent change in the business’s efficiency index required to keep the production index the same?

(A) 100% increase

(B) 50% increase

(C) 30% increase

(D) 30% decrease

(E) 50% decrease

Source: Gmat Hacks 1800

Original question from GMAT Prep:

Quote:

The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present. If the concentration of chemical B is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

A. 100% decrease B. 50% decrease C. 40% decrease D. 40% increase E. 50% increase

Re: In a certain industry, production index x is directly propor [#permalink]
06 Feb 2013, 08:50

Expert's post

Bunuel wrote:

megafan wrote:

In a certain industry, production index x is directly proportional to the square of efficiency index y, and indirectly proportional to investment index z. If a business in this industry halves its investment index, which of the following is closest to the percent change in the business’s efficiency index required to keep the production index the same?

(A) 100% increase

(B) 50% increase

(C) 30% increase

(D) 30% decrease

(E) 50% decrease

Source: Gmat Hacks 1800

Original question from GMAT Prep:

Quote:

The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A present and inversely proportional to the concentration of chemical B present. If the concentration of chemical B is increased by 100 percent, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

A. 100% decrease B. 50% decrease C. 40% decrease D. 40% increase E. 50% increase

Re: In a certain industry, production index x is directly propor [#permalink]
06 Feb 2013, 22:44

Normally i plug numbers to see what happens if certain change applies, i am not sure whether i am doing correct but please comment if anything wrong in my method:

Let say the efficiency index y=10 and investment index=1, so, per question terms, production index x=y^2/x=10^2/1=100; now we change the investment index to half (i assumed that it has been decreased to half since it is not clear in the question) and we have the following: 100=y^2/0.5=(y^2)*2=100 --> y=\sqrt{50} --> equals to about 7, then we can see that efficiency index should be reduced to approximately 30% (from 10 to 7).

I hope that my way of thinking is correct. _________________

If you found my post useful and/or interesting - you are welcome to give kudos!

Re: In a certain industry, production index x is directly [#permalink]
21 Apr 2013, 00:59

Marcab wrote:

The entire expression can be written in the form x=k*y^2/z. When z is halved, the expression becomes x=2k*y^2/z. In order to make this expression equal to its initial form, we need to divide this expression by 2. This can be done by reducingy to y/\sqrt{2}. Therefore percent change= y-y/1.414=>0.414y/1.414. This is almost equal to 30%. Hope this helps.

I used the same method but skipped the calculations since you know that since z is being halved y also has to decrease. There are only 2 options that decrease. If i plug in 50, the condition isn't met and hence 30 is the answer. You should be able to deduce this in less than 30 secs. _________________

You've been walking the ocean's edge, holding up your robes to keep them dry. You must dive naked under, and deeper under, a thousand times deeper! - Rumi

Re: In a certain industry, production index x is directly propor [#permalink]
30 Jul 2014, 01:25

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