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This month's IEEE GlobalSpec newsletter challenge is a classic riddle from Martin Gardner:
If three cats catch three rats in three minutes, how many cats would catch 100 rats in 100 minutes?
Hint: the answer is not "three."
And the answer is:
This is a bit of trick question. The classical answer is that it most likely takes one cat one minute to catch one rat, so the same three cats would catch 100 rats in 100 minutes.
However, this assumes that one cat catches one rat every minute, which is not stated in the question. What if, during the original three minutes, the three cats all concentrate on one rat, then move to the second? It therefore cannot be assumed that one cat can catch one rat in one minute. But even if the cats each focus on a single rat and takes three minutes to catch it, in line with the original premise, it could still be assumed that the three cats take 99 minutes to catch 99 rats.
The last rat poses a problem, however -- there is no assumption of how long it would take the three cats to catch the single rat. If they require the full three minutes of chase, it would take them 102 minutes to catch 100 rats.
The safest answer to the question as stated is that it takes "at least three cats but no more than four" to catch 100 rats in 100 minutes. The most correct answer, however, is that more information is needed on exactly how the cats catch each rat to make a better guess.
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"Almost" Good Answers: