Problem:

Four people need to cross a bridge at night. They have one torch and the bridge is too dangerous to cross without it. Their speeds are 1 minute, 3 minutes, 6 minutes, and 8 minutes. The bridge can only hold two people at a time. What is the fastest way for all four to cross the bridge?

Scroll to the bottom of the page for the answer

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Solution:

The fastest two (1 and 3 minutes) cross first, 1 minute returns with the torch, the slowest two (6 and 8 minutes) cross, then 3 minutes returns with the torch. Finally, the fastest two cross again. Total time = 21 minutes.