GMAT Club Tests | Re: M17-37

If Stephen has to take a lead of 4 km in 20 minutes, it means he has to take a lead of 12 km in 60 minutes (or 1 hour)
(Just multiplied both with 3, to calculate in km/h)

So their relative speed needs to be 12 km/h. Stephen is right now at 80 km/h and Andrew at 72 km/h. Their relative speed as of now is (80-72) = 8 km/h. So Stephen needs to further increase his speed by (12-8) = 4 km/h

Hence option D answer

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