Gregmat GRE mini test 10

GRE Quant Problem Solving
1

Hi in this problem from GRE mini test 10, I don’t understand why the ratio of A/B is 4/5 and not 5/4 when in the picture it’s showing difference in original percentage and new percentage of A is 5? Can anyone please explain the concept properly? Thanks

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2

Hi,
So how this method works is, let’s assume the number 30 as C.
Now in order to obtain C, ratio of A & B mixed should be (B-C):(A-C).
Basically the subtracted numbers are switched to obtain the new ratio.
As this is a shortcut trick the logic behind this is little bit long, I suggest you to keep this 2 steps in mind as they are pretty simple.

3

Thanks! Is there any materials available in Gregmat or elsewhere to understand the concept?

4

Im not sure if Greg has explained this method in his concept classes, but a scholar has explained it step by step in Test Your GRE Quant concepts - 5th July 2022 from 21:25. Logic behind this isn’t cleared but this might help you.