Bridge Crossing Puzzle Solution

 A group of four people has to cross a bridge. It is dark, and they have to light the path with a flashlight. No more than two people can cross the bridge simultaneously, and the group has only one flashlight. It takes different time for the people in the group to cross the bridge: Annie crosses the bridge in 1 minute Bob crosses the bridge in 2 minutes Carl crosses the bridge in 5 minutes Dorothy crosses the bridge in 10 minutes How can the group cross the bridge in 17 minutes?

Solution:

 We shall list the solution in the first column, the time required for each task in the second column, and a running count of the time in the right column. Action Time Required Running Time 1. Bob and Annie cross. 2 minutes 2 minutes 2. Bob comes back across alone. 2 minutes 4 minutes 3. Carl and Dorothy cross. 10 minutes 14 minutes 4. Annie comes back across. 1 minute 15 minutes 5. Annie and Bob head back across. 2 minute 17 minutes

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