<PRE><B>Question 819934</B>
s = d / t
t = d / s
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down:
t = 72/(20 + w)
up:
t = 48/(20 - w)
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72/(20 + w) = 48/(20 - w)
72(20 - w)/(20 - w)(20 + w) - 48(20 + w)/(20 - w)(20 + w) = 0
72(20 - w) - 48(20 + w) = 0(20 - w)(20 + w)
1440 - 72w - 960 - 48w = 0
480 = 120w
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w = 4 mph
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