There are 4 people on one side of the river, let them be A, B, C and D. There is a boat on the same side of the river. A takes 1 minute to row to the other side, B takes 2 minutes, C takes 9 minutes and D takes 10 minutes. Only two people can sit in the boat at one time. Out of the two people who sit on the boat, one who needs more time to row the boat is considered. For example, (A, B) sit on the boat then maximum(1, 2), that is, 2 minutes needed to row to other side. Empty boat can not return to the side where people are standing, hence, one person has to take it back to original side.