A man drives 1 mile to the top of a hill at 15 mph. How fast must he drive 1 mile down the other side to average 30 mph for the 2-mile trip?
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The answer is not 45 mph (with the idea that 15 + 45 / 2 = 30). In that event the man would cover 2 miles in 1/15 + 1/45 of an hour, for an average speed of only 22.5 mph.
In fact he can’t possibly drive fast enough. To average 30 miles per hour, he would have to cover the 2 miles in 1/15 hour, or 4 minutes. But he’s already consumed 4 minutes in driving to the top of the hill (1 mile / 15 miles per hour = 1/15 hour, or 4 minutes). So it can’t be done.
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