A family had to cross a river over a bridge at night. The father can do it in 1 minute, the mother can do it in 2 minutes, their son in 5 minutes and grandma in 10. They have a torch. The bridge can hold only 2 people.
How can they cross the bridge in 17 minutes?
Notices: 1) When there are two people on the bridge they are moving with the speed of the slowest of them. 2) It is not allowed to cross the bridge without the torch, to light from a distance, to carry each other, to throw a torch to each other.
