While working on a case Dr Watson accidentally fell down a 30ft dry wishing well. Sherlock Holmes lowered him down a rope. "

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问题 While working on a case Dr Watson accidentally fell down a 30ft dry wishing well. Sherlock Holmes lowered him down a rope.
    "Can you climb up?" shouted Holmes.
    "I’ll be out before you know it. " came Watson’s reply.
    But the climb wasn’t as easy as Watson had first imagined.  Each hour he managed to climb 3ft--but slipped back 2ft.

    How long did it take Watson to get out?

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答案28 hours

解析 他一小时爬3ft,倒退2ft,就相当于一小时爬1ft,当他爬了27小时后,爬了27ft,那么剩下的3ft正好还需要一个小时,故共需28小时。
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