Once upon a time, there were four friends named A, B, C, and D. They needed to cross a bridge to get to their destination, but the bridge was old and rickety and could only hold two people at a time.
To make things more difficult, the bridge was also very dark and the friends had only one torch between them to light the way. The friends knew that the torch had limited battery life and would only last for 17 minutes before it went out.
Here’s how long it takes each person to cross the bridge:
A can cross the bridge in 1 minute.
B can cross the bridge in 2 minutes.
C can cross the bridge in 5 minutes.
D can cross the bridge in 10 minutes.
The friends knew that when two people crossed the bridge together, they had to move at the pace of the slower person. They also knew that they had to conserve the torch’s battery life as much as possible.
The friends needed to figure out a plan to get everyone across the bridge in the fastest possible time before the torch went out. Can you help them come up with a solution?
Here is a solution for the four friends to cross the bridge in the fastest possible time:
1. A and B cross the bridge together, taking 2 minutes (B is the slower one).
2. A goes back with the torch, taking 1 minute.
3. C and D cross the bridge together, taking 10 minutes (D is the slower one).
4. B goes back with the torch, taking 2 minutes.
5. A and B cross the bridge together, taking 2 minutes (B is the slower one).
6.Total time taken so far is 2 + 1 + 10 + 2 + 2 = 17 minutes, which is the torch’s battery life.
7. C goes back with the torch, taking 5 minutes.
8. A and B cross the bridge together, taking 2 minutes (B is the slower one).
9. Total time taken is 17 + 5 + 2 = 24 minutes.
Thus, the four friends can cross the bridge in a total of 24 minutes by following this plan.