Flying east between two cities a planes speed is 380 mph On

Flying east between two cities, a plane\'s speed is 380 mph. On the return trip it flies at 420 mph. Find the average speed over the whole round trip. (NO, the answer is not 400 mph.)

Solution

d= rt --> t = d/r
t = x/10 + x/8 = 9x/40
r = 2d/t = 2x/t --- 2x for the round trip
r = (2x/1)/(9x/40) = 80x/9x
r = 80/9 = 8.888... mph
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It\'s not the average, 9 mph, because he spent more time going 8 mph than he did going 10 mph.