Three candles in increasing height, from left to right.

Your turkey needs exactly 6 hours to slow roast. Alas, you have no timer, just three candles: one that burns in 3 hours, one that burns in 4 hours and another that burns in 5 hours.

How can you measure 6 hours using these candles, putting the roast in the oven at the same time as you light the first candle?

This puzzle was published on the New Scientist.

Solution

Here’s one way. Light all three candles. When the 3-hour candle burns out, blow out the 4-hour candle. When the 5-hour candle burns out, re-light the 4-hour candle for its final hour.

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