The solution of four-person bridge problem

Problem: There are four people in the evening to cross the bridge, only a flashlight, each bridge needs a flashlight, each time up to two people, including a bridge to 1 minutes, B to 2 minutes, C to 5 minutes, ding to 10 minutes. To find the shortest bridge time.

Answer: A b before, spents 2 minutes, and then a flashlight to send back, total time 3 minutes, and then in the past, the total use of 13 minutes, and then a flashlight sent back, the total use of time-varying for 15 minutes, and then a B together in the past, four people have to the opposite shore, total time 17 minutes.