Question 383994
It's not 50 mph.  More time is spent moving at 40 mph than moving at 60 mph.  It's always less than the average of the 2 speeds, unless they're equal.
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d = rt
t = d/r
d = the distance, so t = 2d/r for the round trip
2t = d/60 + d/40 = 100d/2400 = d/24 (total time)
t = d/48
r = 48 mph