Clock Struck 21

One minute ago, a clock that strikes only the hours just finished striking an hour. If the clock has struck a total of 21 in the last 2.5 hours, what hour did it just finish striking?


8 o’clock. If the clock just finished striking, it has struck three consecutive hours in the last 2.5 hours: the one it just struck, the one it struck an hour ago, and the one it struck two hours ago. The sum is 6 + 7 + 8 = 21.

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