Bridge Riddle
Adam, Bob, Clair and Dave are out walking: They come to rickety old wooden bridge. The bridge is weak and only able to carry the weight of two of them at a time. Because they are in a rush and the light is fading they must cross in the minimum time possible and must carry a torch (flashlight,) on each crossing.
They only have one torch and it can't be thrown. Because of their different fitness levels and some minor injuries they can all cross at different speeds. Adam can cross in 1 minute, Bob in 2 minutes, Clair in 5 minutes and Dave in 10 minutes.
Adam, the brains of the group thinks for a moment and declares that the crossing can be completed in minimum time possible, what is that time ?
The answer is 17.
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1 solution
We can save time in this problem by having the fastest people passing the torch, and the slowest people cross together.
First, Adam and Bob cross together as the two fastest. Adam has to slow down for Bob and by the time they have reached the other end of the bridge, 2 minutes have passed.
Then, Adam runs back to Clair and Dave in 1 minute. 3 minutes pass total so far. You pass the torch to Clair and Dave and they cross together in 10 minutes to Bob. 13 Minutes total so far.
Bob as the fastest of the three on the other end grabs the torch and runs to you in 2 minutes. You come along on the last trip and take 2 more minutes, summing the minimum amount of time taken to be 17 minutes.