Fuses: Difference between revisions
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You have two fuses, each twelve inches long. Each fuse burns in exactly one hour, but does not necessarily burn at a uniform rate. Also, the two fuses do not necessarily burn at the same rate over corresponding segments, but a given segment on a given fuse burns in the same amount of time in either direction. How do you use these two fuses to time 15 | You have two fuses, each twelve inches long. Each fuse burns in exactly one hour, but does not necessarily burn at a uniform rate. Also, the two fuses do not necessarily burn at the same rate over corresponding segments, but a given segment on a given fuse burns in the same amount of time in either direction. How do you use these two fuses to time 15 | ||
minutes? (You are allowed as much time to prepare as you wish.) | minutes? (You are allowed as much time to prepare as you wish.) | ||
==Extension== | ==Extension== | ||
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{{Problem Solving}} | {{Problem Solving}} | ||
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Revision as of 09:24, 6 December 2010
A very nice puzzle from Problem Solving by Thomas DeFranco and Charles Vinsonhaler.
Puzzle
You have two fuses, each twelve inches long. Each fuse burns in exactly one hour, but does not necessarily burn at a uniform rate. Also, the two fuses do not necessarily burn at the same rate over corresponding segments, but a given segment on a given fuse burns in the same amount of time in either direction. How do you use these two fuses to time 15 minutes? (You are allowed as much time to prepare as you wish.)
Extension
How do you time 15 minutes using only one fuse?
Help
Hint
Given one fuse, can you think of a way to time exactly 30 minutes?
Solution
Take one of the fuses, fuse 1, and bend it so that it forms a loop. Take the other fuse, fuse 2, and place it so that one end touches both ends of fuse 1. Light this area where all three ends meet. Then fuse 1 will finish burning in exactly 30 minutes, at which time fuse 2 has another 30 minutes remaining. Thus we should light the other end of fuse 2 at this time. From this point on it will take 15 minutes for what remains of fuse 2 to finish burning.
References
Problem Solving by Thomas DeFranco and Charles Vinsonhaler.