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?"