Maths

There are four friends Rachel, Ross, Monica and Chandler who want to cross a bridge on the river at night. But they have only one torch with them. Only two people can cross the bridge at a time and everyone requires a different time to cross the bridge. The battery life of torch is only 15 minutes and the time required by the friends: Rachels needs 1 minute, Ross needs 2 minutes, while Monica requires 5 minutes and Chandler requires 8 minutes. How can they all cross the bridge before the torch dies off?

$$ x = {-b \pm \sqrt{b^2-4ac} \over 2a}. $$