World Maths Day

Can you solve the problem?
Four friends need to cross a bridge.
They start on the same side of the bridge.
A maximum of two people can cross at any time.
It is night and they have just one lamp.
People that cross the bridge must carry the lamp to see the way.
A pair must walk together at the rate of the slower person:
Rachel: - takes 1 minute to cross
Ben: - takes 2 minutes to cross
George: - takes 7 minutes to cross
Yvonne: - takes 10 minutes to cross
What is the fastest solution?