FILE No.76Unsolved
A bomb will go off after exactly 45 minutes, and you must time it. There is no clock. All you have are two ropes that each burn for exactly 60 minutes and a lighter.
However, the ropes are uneven, so half the length does not burn in 30 minutes. You can light a rope at either end, or both.
How do you measure 45 minutes?
If you get stuck, open the hints one at a time, in order.
Even an uneven rope takes exactly 60 minutes to burn completely.
How long does a rope take if lit from both ends?
How could you use what is left of rope B?
How do you measure 45 minutes?
Choose the answer closest to the truth.
Light both ends of rope A and one end of rope B at the same time; when A burns out, light the other end of B
The answer is: light both ends of rope A and one end of rope B together; when A burns out, light the other end of B.
Lit at both ends, even an uneven rope burns out in exactly half the time, 30 minutes. At that moment rope B, burning from one end, has exactly 30 minutes of burn left. Lighting its other end makes it burn out in 15 more minutes. 30 + 15 = 45 minutes.
People assume half the length takes 30 minutes, so they think uneven ropes cannot work.
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