One family wants to get through a tunnel.
Dad can make it in 1 minute, mom in 2 minutes, son in 4 and daughter in 5 minutes. Unfortunately, not more than two persons can go through the narrow tunnel at one time, moving at the speed of the slower one.
Can they all make it to the other side if they have a torch that lasts only 12 minutes and they are afraid of the dark?
First mom and dad – 2 minutes.
Dad comes back – 3 minutes, both children go to mom – 8 minutes.
Mom comes to dad – 10 minutes and they both get to their children – 12 minutes. 🙂