A family wants to get through a tunnel at night.
Dad can make it in 1 minute, mom in 2 minutes, son in 4 minutes and daughter in 5 minutes.
Unfortunately, not more than two people can go through the narrow tunnel at one time, moving at the speed of the slower one.
They have only one torch and it only has enough light left for 12 minutes.
The tunnel is too dangerous to cross without a torch.
How can they all cross the tunnel in 12 minutes?
1st pass, the father and mother go together (2 minutes)
2nd pass, the father goes back (1 minute)
3rd pass, the son and daughter go together (5 minutes)
4th pass, the mother goes back (2 minutes)
5th and final pass, the father and mother go together (2 minutes)