There is a bridge.
The bridge can only support 2 people at a time.
There are 4 people.
Each person takes a different amount of time to cross the bridge:
Person 1 = 1 minute
Person 2 = 2 minutes
Person 3 = 5 minutes
Person 4 = 10 minutes
It is dark, and there is only 1 flashlight. Thus, you need the flashlight to cross the bridge. That means people must travel in pairs.
What is the least amount of time it takes for all 4 people to cross?
Note: Nothing strange – you can’t throw the flashlight back across, you can’t ‘shine the light ahead’ for one of the people, etc. It’s simple.