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100 points for $49 worth of Veritas practice GMATs FREE VERITAS PRACTICE GMAT EXAMS Earn 10 Points Per Post Earn 10 Points Per Thanks Earn 10 Points Per Upvote ## In a certain economy, C represents the total amount of con tagged by: vinni.k ##### This topic has 3 expert replies and 1 member reply ## In a certain economy, C represents the total amount of con In a certain economy, C represents the total amount of consumption in millions of dollars, Y represents the total national income in millions of dollars, and the relationship between these two values is given by the equation C=90+ 9Y/11. If the total amount of consumption in the economy increases by 99 million dollars, what is the increase in the total national income, in millions of dollars? A.11 B.22 C.99 D.121 E.171 OA is D Can you please give 2 or 3 approaches on how to handle this question ? My approach is taking about 3.5 to 4 mins on this question. Thanks ### GMAT/MBA Expert Legendary Member Joined 14 Jan 2015 Posted: 2667 messages Followed by: 122 members Upvotes: 1153 GMAT Score: 770 Top Reply vinni.k wrote: In a certain economy, C represents the total amount of consumption in millions of dollars, Y represents the total national income in millions of dollars, and the relationship between these two values is given by the equation C=90+ 9Y/11. If the total amount of consumption in the economy increases by 99 million dollars, what is the increase in the total national income, in millions of dollars? A.11 B.22 C.99 D.121 E.171 OA is D Can you please give 2 or 3 approaches on how to handle this question ? My approach is taking about 3.5 to 4 mins on this question. Thanks Say the initial consumption was 0. We can find the initial national income by solving 0 = 90 + 9y/11; and y = -110 If the new consumption is 99, we can find the new national income by solving 99 = 90 + 9y/11; now y = 11 So the national income increased from -110 to 11, for a net increase of 121. The answer is D _________________ Veritas Prep | GMAT Instructor Veritas Prep Reviews Save$100 off any live Veritas Prep GMAT Course

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Hi vinni.k,

Here's a bit more explanation on the approach that I used:

9Y/11 will equal some value. To increase that value, we clearly have to increase the value of Y.

Regardless of what the 'starting value' is for Y, IF we increase Y by 11, then the INCREASE to 9Y/11 will be.... 9(11)/11 = 9

For example. IF Y = 0, then 9(0)/11 = 0
Increasing Y by 11 gives us 9(11)/11 = 9

Based on the wording of the prompt, we need to increase the value of 9Y/11 by 99... and that is '11 times' more than the example that we just dealt with. Thus, we have to increase Y by 11 and THEN multiply that number by 11.... (+11)(11) = 121.

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### GMAT/MBA Expert

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Hi vinni.k,

We're given the equation C = 90 + 9Y/11. With this relationship, if we increase/decrease one of the variables, then we will have to also increase/decrease the other variable to properly 'compensate' for the change. This question can be solved rather easily by TESTing VALUES (the approach David used). You can also solve it with a bit of arithmetic and some logic.

If the consumption (C) increases by 99, then we to increase the 'right side' of the equation 99 also. We can't change the "90", so we have to increase the value of Y so that 9Y/11 becomes '99 more' than it used to be. You can do this math in 2 small steps:

First, to cancel out the denominator, we can make Y=11. this gives us 99/11 = 9. Second, since we're dealing with multiplication, to raise 9 to 99, we need to multiply by 11. Thus, we're increasing Y by 11 and then multiplying by 11. This is the equivalent of (11)(11) = 121.

GMAT assassins aren't born, they're made,
Rich

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Rich,
thanks for your approach too, but this is what i was able to make from the first few statements
9y/11 = 99 . After solving this gives me 121; however, i am not able to understand the following explanation.

Rich.C@EMPOWERgmat.com wrote:
First, to cancel out the denominator, we can make Y=11. this gives us 99/11 = 9. Second, since we're dealing with multiplication, to raise 9 to 99, we need to multiply by 11. Thus, we're increasing Y by 11 and then multiplying by 11. This is the equivalent of (11)(11) = 121.

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