Mischa drove from \(A\) to \(B,\) a distance of \(100\) miles, at an average speed of \(50\) miles per hour, and then

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Mischa drove from \(A\) to \(B,\) a distance of \(100\) miles, at an average speed of \(50\) miles per hour, and then back from \(B\) to \(A\) along the same route at an average speed of \(m\) miles per hour. What is the value of \(m?\)

(1) Mischa’s average speed for the entire round trip, excluding any time he spent at point \(B,\) was \(\dfrac{m+50}2\) miles per hour.

(2) If Mischa’s average speed from \(B\) to \(A\) had been \(20\%\) slower, the total time for the entire round trip, excluding any time Mischa spent at point \(B,\) would have been \(12.5\%\) greater.

Answer: D

Source: Manhattan GMAT