Required Mathematical Intelligence

Chapter 150: Crossing the Bridge
Today, there are four people a, b, c, and d who will go from the left side of the bridge to the right side at night.Only two people can walk on this bridge at a time, and there is only one flashlight. You must use a flashlight when crossing the bridge.

The fastest time for four people to cross the bridge is as follows: a 2 minutes; b 3 minutes; c 8 minutes; d 10 minutes.People who walk fast have to wait for people who walk slowly. How can everyone cross the bridge at 21 minutes?

[Answer: First, a and b cross the bridge together, then leave b on the other side, and a returns alone. After a returns, give the flashlight to c and d, let c and d cross the bridge together, after c and d reach the opposite bank, give the flashlight to b, let b bring the flashlight back, and finally a and b cross the bridge together again.Then the required time is: 3 2 10 3 3=21 minutes. ]
(End of this chapter)