Four friends A, B, C, D need to cross a bridge

Four friends A, B, C, D Need to cross a bridge.
A maximum of 2 peoples can cross at a time.
It is night and they have just 1 lamp.
People that cross the bridge must carry the lamp to see the way.
A pair must walk together at the speed of slower person.

Speeds of:
A: 1 minute to cross bridge
B: 2 minutes to cross bridge
C: 7 minutes to cross bridge
D: 10 minutes to cross bridge

Now the question is...
"What is the total minimum time required by all 4 friends to cross the bridge?"

Click here for Answer

Total = 21 Minutes