You have a piece of string which when set on fire from one end takes 120 minutes to burn completely. It has a property that the time taken to burn a distance D from one end is the same as the time taken to burn a distance D from the other end. The rate of burning is not uniform – different parts of the string will take different amounts of time to burn.
You need to boil an egg for exactly 15 minutes using the string as a timer. How will you do it?
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The answer to the puzzle is :
Cut the string from the centre. Now you have two parts each of which will take 60 minutes to burn completely, if set on fire from one end. This is because the string is symmetrical around its centre as stated in the puzzle.
Now set fire to one of the pieces of the string from both ends and at the same time set fire to the other piece only from one end. The string that is burning from both ends will take 30 minutes to burn completely – it is burning at twice the rate.
At the end of 30 minutes set fire to the other end of the string that is burning only from one end and at the same time start boiling the egg. This piece of string had 30 minutes left to burn, but since now it is lit from both ends, it will take only 15 minutes to burn completely.
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