A bridge is about to collapse. There are four people P, Q, R and S on one of the side. Before the bridge collapses, they want to cross it. Now since the bridge is too weak, it can only stand the weight of two people at a time. Also, it is night time and nothing is visible. They have just one torch with them.

Now P takes one minute to cross the bridge, Q takes two minutes to cross, R takes five minutes to cross and S takes ten minutes to cross.

The bridge will collapse in seventeen minutes. How will they be able to cross the bridge before it collapses?

P and Q cross the bridge first taking two minutes.

P comes back with which makes the total time three minutes.

R and S cross this time making the total time thirteen minutes.

Q comes back with the torch making the total time fifteen minutes.

Now P and Q cross the bridge finally taking two more minutes and making the total time seventeen minutes.

Thus all were able to reach before the bridge collapsed.