Hard question from the gmat book

Hi,

I can’t seem to solve this task using choosing numbers. The explanation they gave below is quite abstruse and involves some pretty advanced algebra. The correct answer is D. Though I don’t know how I could solve it using GregMat methods. Could anyone take a look?

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Look at the 2nd answer by BrentGMATPrepNow. He shows how to solve by choosing numbers. That would be the GregMat approach to this problem IMO

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