One 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?
Answer
Let:
- Dad = F (1)
- Mom = M (2)
- Son = S (4)
- Daughter = D (5)
This is how they cross the tunnel:
- F,M → (2)
- ← F (1)
- S,D → (5)
- ← M (2)
- F,M → (2)
The time in brackets () indicate the time taken to cross the tunnel. If you sum them up, it will add up to 12 minutes.