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You are given two pieces of identical string, each exactly 1 meter long, along with a box of matches. Each piece of string, burns for exactly one hour, albeit not constantly (ie one section of each string burns longer then another). You are then removed of any devices that enable you to tell the time. Your task, is to figure out how to measure 45 minutes, using just the two pieces of string!
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The answers to this were both wrong. If you burn the strings at opposite ends, then you will have two different pieces of string after they have "crossed". You can't measure 15 minutes with different strings.
Also, if you make a makeshift sundial, 45 minutes will have already passed and you're really taking 1:45 to do it. Besides, it's not exact.
Here's the solution:
Burn string A at both ends, and at the same time, burn string B at one end. String A will completely burn out in 30 minutes. At this time, light the other end of string B (which still has 30 minutes left). Now, the string will completely burn out in 15 minutes, for a grand total of 45 minutes.