4 men must cross a bridge at night.
It is pitch black.
They have only one flashlight.
They each walk at different speeds.
The first can walk the bridge (one direction) in 20 minutes.
The second in 15.
The third in 5 and the fourth in 1.
The bridge is so narrow and weak that it can only accommodate 2 men at a time.
The bridge will blow up in 40 minutes.
Can you get them across in time?
Example:
Man 1 and Man 4 cross in 20 minutes.
Man 4 comes back with the light, 1 minute.
Man 2 and man 4 cross in 15 minutes.
Man 4 comes back with the light, 1 minute.
Man 3 and man 4 cross in 5 minutes.
Done.
Total time is 42 minutes. They did not make it 馃檨