This is a very nice puzzle from fivethirtyeight.com (thanks to Suman Saraf for pointing this site to me).
It’s your 30th birthday, and your friends got you a cake with 30 lit candles. You try to blow them out, but each time you blow you successfully extinguish a random number of candles, between one and the number that remain lit. How many blows will it take, on average, to extinguish them all?
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