Source: Official Guide for GMAT Review 2016 Problem Solving; #66

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# If x/y =c/d and d/c = b/a , which of the

If x/y =c/d and d/c = b/a , which of the following must be true?

### 1 Explanation

1

ROHAN MATHEW

The answer has everything in terms of x,y,a and b. So form an equation with these terms from the question.
Now the second equation d/c=b/a can be rewritten as c/d=a/b.Notice that both the first and second equation has c/d in common now.So we can conclude that x/y=a/b
This equation can be rewritten in terms of answer choice (I) and (II) by cross multiplying.But notice that answer choice(III) can never be derived from x/y=a/b
Therefore,the correct answer is option C

Mar 2, 2016 • Comment

Cydney Seigerman, Magoosh Tutor

Thanks for sharing your solution to this question :)

Mar 4, 2016 • Reply

Gregory J Broder

Hi Cydney, Can you not just do simple smart numbers for this question?

Aug 31, 2016 • Reply

Sam Kinsman

Good question Rohan! You can definitely do that. That is, you can pick numbers for each variable, and then test each answer choice to see if it's true. Just be careful when you're picking the numbers: you need to make sure that both of the initial equations (x/y =c/d and d/c = b/a) are true when we plug in the numbers.

Sep 1, 2016 • Reply

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