In the figure, lines AB, CD and EF are parallel, and lines B

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[GMAT math practice question]

In the figure, lines AB, CD and EF are parallel, and lines BC and DE are parallel. What is the length of CD?



1) AB=3
2) EF=12

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Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

Triangles ABC and CDE are similar, and triangles BDC and DFE are similar. So, we can set up the proportion AB:CD = CD:EF.
Since CD^2 = AB*EF, conditions 1) and 2) together give us sufficient information to calculate the value of CD.

Therefore, C is the answer.
Answer: C