Big Book segment questoin

GRE Quant Problem Solving
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I am very confused on how to solve this question. I originally put 5 as the answer, as per the beginning of Greg’s explanation of the video: 26-21 is 5 for XY. However, when you keep reading, you realize that YZ needs to be twice WX.

The answer is D, 16. I get the rest of his backsolving answer (here: Test 1 Harder Quant Walkthrough - GregMat) but don’t get the logic.

How is it possible that XY could be 16 when the overlap is between 26 and 21? given thats the overlap, doesnt it have to be 5?

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XY could be any of the choices and the diagram would still make sense. Try visualizing that and perhaps that could sort out some of your question.

As for how to solve it, why don’t you just use algebra?

Let WX = x, XY = 21 - x = t, and YZ = 26 - t. You know that “YZ is twice WX”, so that gives 2x = 26 - t. Moreover, XY is represented by two variables so 21 - x = t. Can you take it from here?