Trivia

Subject:
Friday Puzzler -- Lighthouses

Alex Y
There are three light houses on a shore. They flash with different frequencies; one is 3 seconds on and 3 off, another is 4 on and 4 off, and the third is 5 on and 5 off.

If they all turn on at time zero, when will it next be dark (none on). When will they next all three come on at the same time?

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