Gateways To Joy
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Puzzles
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Set 2
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Forty-Five Minutes
Forty-Five Minutes
12 Sep 2008
Puzzle

How do we measure forty-five minutes using two identical wires, each of which takes an hour to burn. We have matchsticks with us. The wires burn non-uniformly. So, for example, the two halves of a wire might burn in 10 minute and 50 minutes respectively.

Source

Heard from Sridhar Rajagopalan in 1998.

Solution

Light three out of four ends. When two ends meet, light the fourth!

Followup

How would you measure 20 minutes, 40 minutes, and so on? In general, for measuring 60 * (a / b) minutes, see a short article by Adam Costello.

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