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:
Mary: – takes 1 minute to cross
Joseph: – takes 2 minutes to cross
Andrea: – takes 7 minutes to cross
Benedict: – takes 10 minutes to cross
What is the minimum amount of time the four friends need to crosso the bridge? What is the strategy?
Type in the solution here _____ (in minutes).