Crossing the Bridge

Contributed by Mike Hesser: There are four people of different age. It's dark and foggy and they want to cross a bridge. The bridge is a very old one which can only carry two persons at once. In addition the people have only one lamp which is necessary to cross the bridge safely.

Because the people are of different age, they need different times to get over the bridge. The youngest needs 5 minutes, the next one 10 minutes, the old man 20 and his wife 25 minutes.

In which combination is it possible that all four people can cross the bridge within 60 minutes? (Remember: only two persons at once, and they need the lamp.)

Think it over!

Then check the answer here.

Published in Njet Problem! collection 1996-09-06.