Brainteaser #27 – Burning Rope

brainteasers

Locked in a cell deep in the jungle you are given a traditional tribal brainteaser.

If you solve it, you will be set free!

If you fail, you will be cast into a pit of vipers.

Here it is:

You are given two ropes, coated in petrol, and a lighter.

Each rope will burn out in exactly 1 hour. However, they will not burn at a regular pace. They may slow down or speed up, but, even so, they will burn fully within 1 hour.

By igniting the ropes, how can you measure exactly 45 minutes?

Of course, you cannot simply measure 3/4 down the rope as they burn at different rates, and thus, this would not be 45 minutes.

Show Answer

Given that either rope will take 1 hour to burn, you know that by lighting both ends, the rope will burn out in 30 minutes. This is because lighting the rope at both ends will double the rate at which it burns.

So, first, you light a rope at both ends, and immediately after, you light the other rope at one end only.

When the first rope burns totally out, in 30 minutes’ time, you immediately light the other end of the remaining rope.

The remaining rope will be 30 minutes through burning. By lighting its other end, you double the rate at which it burns, meaning it will now have 15 minutes to burn and not 30 minutes.

Once the remaining rope burns up in 15 minutes time, the 45 minutes is up.

Congratulations!

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