All together now
Abe volunteers at the hospital every 5th night, Brian every 7th night, and Cliff every 9th night. They all start on the same night.
How many nights will elapse before they again all volunteer together on the same night?
The answer is 315.
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1 solution
Moderator note:
Simple standard approach.
Since 5, 7 and 9 are all mutually coprime, it will take 5 * 7 * 9 = 315 nights before they all meet at the hospital again.