Monday Morning Braintease (the original)O
Four bankers, Al, Bob, Cal and Dave need to get across a river. The only way to cross the river is by an old bridge, which holds at most 2 people at a time. Being dark, they can't cross the bridge without a flashlight, of which they only have one. So each pair can only walk at the speed of the slower person. They need to get all of them across to the other side as quickly as possible. Al is the slowest and takes 12 minutes to cross; Bob takes 5 minutes; Cal takes 3 minutes; and Dave takes 1 minute.
What is the minimum time to get all of them across to the other side?
Show thought process for SBs.