Question

A man weighs 78 kg and his two sons weighing 45 kg each were stranded on a river crossing. All they had to cross the river was a small boat, but it could only hold 90 kg. How did the man and his sons cross the river?

Answer 1

The two sons cross the river. 
One son comes back with the boat.
The man crosses the river.
The other son goes back with the boat.
The two sons cross the river.