1 Torch, 1 Bridge and 4 Persons | Puzzle 3

Question

There are 4 persons A, B, C, and D who want to cross a bridge in the night time.

A takes 1 minute to cross the bridge.

B takes 2 minutes to cross the bridge.

C takes 5 minutes to cross the bridge.

D takes 8 minutes to cross the bridge.

They were provided with only one torch and the bridge cannot be crossed without the torch. There cannot be more than two persons on the bridge at any time. And when two people cross the bridge together, they must move at the slower person’s pace

What is the minimum time required to cross the bridge?

Solution

The answer is 15 minutes

Firstly, A and B cross the bridge (2 minutes). A comes back (1 minute). Total Time taken is 3 minutes. Now B is on the other side.

C and D cross the bridge (8 minutes). B comes back (2 minutes). Time taken 8 + 2 = 10 minutes. Now C and D are on the other side.

A and B cross the bridge. Time taken is 2 minutes. All are on the other side.

Hence, Total time spent is 3 + 10 + 2 = 15 minutes.

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