crossing the bridge
Four people need to cross a rickety bridge at night. Unfortunately, they have only one torch and the bridge is too dangerous to cross without one. The bridge is only strong enough to support two people at a time. Not all people take the same time to cross the bridge. Times for each person: 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the bridge?
The answer is 17.
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First 1 & 2 crosses the bridge here 2 mins lapsed then one comes back so one min lapsed......now a total of 3 mins lapsed......now 10 & 7 crosses the bridge here 10 mins lapsed then 2 on the other side will come to this side so 2 mins lapsed......now a total of 3+10+2 =15 mins lapsed........now 1 & 2 will again cross the bridge so 2 mins will be lapsed.......so a total of 15+2 = 17 mins will be lapsed for all to cross the bridge.