Four people arrive at a river with a narrow bridge that can only hold two people at a time. It's nighttime and they have one torch that has to be used when crossing the bridge. Person A can cross the bridge in one minute, B in two minutes, C in five minutes, and D in eight minutes. When two people cross the bridge together, they must move at the slower person's pace. Can they all get across the bridge in 15 minutes or less?
Answer: Yes, they can cross in exactly 15 minutes.
Explanation: The group of four must follow these steps.
1. First, A and B cross the bridge and A brings the light back. This takes 3 minutes.
2. Next, C and D cross and B brings the light back. This takes another 10 minutes.
3. Finally, A and B cross again. This takes another 2 minutes.
1 start both timers
2. when the 7 min timer runs out, start the egg, there are 4 mins left in the 11 min timer
3. when those 4 mins run out restart the 11 min timer