You have two ropes that each burn completely in 60 seconds, but they don't burn at a constant rate. They burn unevenly. Given a match and the two ropes, how would you measure 45 seconds?

Question

Accepted Answer

Light the first rope from both ends and at the same time light the second rope from one end.

The first rope will burn completely in 30 seconds since burning from both ends halves the burn time (even though it burns unevenly, it will still fully burn in half the time).
As soon as the first rope finishes burning, light the second end of the second rope (which has been burning for 30 seconds).

Since the second rope had already burned for 30 seconds, half of it is left. By lighting it from the other end, this remaining half will burn in 15 seconds.

Thus, the total time for both steps is 30 seconds (first rope) + 15 seconds (second rope), which gives you 45 seconds.

You have two ropes that each burn completely in 60 seconds, but they don't burn at a constant rate. They burn unevenly. Given a match and the two ropes, how would you measure 45 seconds?

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