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?"