The bridge will collapse in 17 minutes! Four people need to cross the bridge before it collapses. It is a dark night and they have only one flashlight among them.
Only two people can cross at a time.
- Alice takes one minute to cross.
- Bob takes two minutes.
- Carol takes five minutes
- Dave takes 10 minutes to cross.
The trick to this puzzle is to get the slowest members of the group to cross only once together, while the fastest members cross back and forth multiple times.
- Alice and Bob cross first using up 2 minutes.
- Alice comes back making it 3.
- Carol and Dave cross together making it 13 minutes.
- Then Bob crosses back, making it 15 minutes.
- Finally, Alice and Bob cross together to make it a total of 17 minutes.