A+night+crossing...

A night time problem...

There are four people wishing to cross a bridge that will only support two at a time.

It is night and they have 1 lantern that must be used for any crossing because of the state of the bridge.

Person A can cross in 1minute B can cross in 2mins, C can cross in 5mins and D in 10minutes.

If they cross in pairs then the speed is at the pace of the slowest. What is the shortest possible time for all to cross the bridge?

e.g. If A and D cross it will take 10mins then another 1min for A to return.

There is an answer below 19mins.