BRIDGE EQUATION

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
The second fastest solution gets the friends across in 21 minutes.
The fastest takes 17 minutes. Can you work out how it is done?

If I remember I'll give you the answer tomorrow, afraid there's no prize for anyone who works it out save being a cleverer than the rest of us.