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February 21, 2018, 05:12:36 PM
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: Cross the bridge with a torch!  ( 3143 )
« : May 17, 2007, 09:51:50 PM From Prateek»



Cross the bridge with a torch!


Four people, A, B, C, and D, are on one side of a bridge, and they all want to cross the bridge. However, it's late at night, so you can't cross without a flashlight. They only have one flashlight. Also, the bridge is only strong enough to support the weight of two people at once. The four people all walk at different speeds: A takes 1 minute to cross the bridge, B takes 2 minutes, C takes 5 minutes, and D takes 10 minutes. When two people cross together, sharing the flashlight, they walk at the slower person's rate. How quickly can the four cross the bridge?

PS: Asked in many interviews, including Microsoft and Talisma!

 
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« #1 : May 26, 2007, 12:33:50 AM From spazinvaders»

They can cross in 17 minutes.
Let the persons be A,B,C,D with time intervals needed as 1,2,5,10 repectively.
First A&B==>2 minutes
then A will be back==>1 minute
then C&D goes==>10 minutes
now B will go back==>2 minutes
now A&B goes==>2 minutes
thus 2+1+10+2+2=17 minutes
« #2 : May 28, 2007, 01:37:25 PM From Prateek»

Right answer Spazinvaders.
It is one of those puzzles which are difficult to solve. But once understood difficult to forget. :)
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