(21 votes, average: 3.52 out of 5)You are trying to cook an egg for exactly fifteen minutes, but instead of a timer, you are given two ropes which burn for exactly 1 hour each. The ropes, however, are of uneven densities – i e , half the rope length-wise might take only two minutes to burn. How can you cook the egg for exactly fifteen minutes?
You can measure 1 hour by burning any rope from one end, now if we burn the rope from both the ends, it will take half of the time, i.e. 30 minutes, thus we can measure 30 minutes by burning the rope from two ends.
But we need to measure 15 minutes, we can measure 15 minutes if we burn a rope from two ends, which generally take 30 minutes in burning from one end. In order to achieve that we can use second rope given to us.
Steps to measure 15 minutes
- Light Rope #1 from both ends.
- At the same time light Rope #2 from one end.
- When Rope #1 burns fully, it will be 30 minutes and Rope #2 will take 30 more minutes to burn, light the other end of Rope #2 at this time, thus it will take 15 minutes to burn.
- Start cooking your egg at the same point.
- Stop cooking when Rope #2 burns fully.
first you twist the rope in one like twisted pair cable and then fold it from mid of that twisted pair cable and then burn it from both side and start cooking egg exact after 15 min rope is burned fully and egg will be also cooked ..
it cant be determined by this since the densities are different.. for example divide the rope in ten parts of equal length where 8 parts burn in 10min and rest two parts in 50min, if this two parts are next each other, 6/8th of the rope would be burnt in 10mins eventhough u light both ends. The point is 30minutes burining does not gaurantee half the length.. Puzzle is flawed.. with the egg burnt
why cant we first fold the rope joining two end and then light it from both the ends ?
As we are interested in how much time it is burnt rather than what part in which time.
Burn Rope A from both end and rope B from one end. When The rope A is fully burnt we have rope B for 30 Mins. So Now start cooking and burn rope B from both end.So it will burn in 15 mins.
Since the rope is of unequal densities, when rope A is fully burnt in 30 minutes,rope B would have burnt by 30 mins that can be half, more than half or less than is also possible since the rope is of unequal density i.e the the length of rope burnt in 30 minutes is not neccessarily half,hence you cant guarantee the solution by your approach.
why cant we first fold the rope joining two end and then light it from both the ends ?
Because densitity of rope is not same.
We light rope A from one end and rope B from both ends simultaneously.
When rope B is fully burned (it means that half an hour is passed and rope A has only one hour left) we light the other end of rope A immediately and of course start cooking the egg.
When rope A is fully burned (it means that fifteen minutes is passed from the time we started cooking the egg) your meal is ready…! 😉
It means 30 minutes is left, not one hour. Correct your sentence.