The puzzle:

There is a bridge over a river that is going to blow up in 17 minutes. Four people want to cross the bridge in time before it blows up. Each person has different physical ailments and thus takes different times to cross the bridge.

Crossing constraints:

Further, it is dark and since they have only one flashlight, they must cross the bridge as follows: two persons cross the bridge, and one comes back with the flashlight, then two more persons cross the bridge and one comes back and so on until all four are across the bridge.

Each of the four persons take the following times to cross the bridge one way:

A: 10 minutes

B: 5 minutes

C: 2 minutes

D: 1 minute

When two persons cross the bridge, the time it takes is equal to the time it takes the slower person to cross. For example, if A and C cross together, it will take them 10 minutes to cross but if B and D cross it takes 5 minutes.

For example, if A and D first cross the bridge first and D comes back with the flashlight, then it takes a total of 10 + 1 = 11 minutes.

The question is: how would you schedule the bridge crossings so that all four can cross the bridge given the crossing constraints within 17 or less minutes.

Spoiler for Solution