4 people need to cross a bridge before it collapese,  they have seventeen minutues befor the collapse. there is only one flashlight and each must have the light to cross. the bridge can only hold 2 people at a time ,can you save them ？

1. 1 old man （need 10 minute to cross）

2. 40  year old woman （need 5 minute to cross）

3. 20 year old （need 2 minute to cross）