In the puzzle Four people on a Bridge we met four people who needed to cross a bridge at night. In this puzzle, there are five people who must cross two sequential bridges at night. As in the original puzzle, there are some hindrances:

The bridges can only support two people crossing at a time.

Each person has a different speed in which they can cross: 10 minutes, 7 minutes, 5 minutes, 2 minutes, and 1 minute.

They have only two flashlights to share among them.

What is the shortest amount of time it will take for all five people to cross both bridges?

Required shortest amount of time is 23 Minutes
Explanation:
Let the persons able to cross in 10 Min,7 Min,5 Min 2Min, 1Min. be denoted by N10,N7,N5,N5,N2,N1. Then,
i) N1 and N2 cross over (2 Min);  
ii) N2 comes back but N1 stays (2 Minutes).
iii) N7 and N10 cross over ( 10 Minutes);
 iv) N1 comes back (1 minute)
v) N1 and N5 cross over ( 5 Minutes);
vi) N1 comes back ( 1 Minute)
vii) N1 and N2 cross over ( 2 Minutes)
so that ,total time taken = (2+2+10+1+5+1+2)=23 Minutes

 

Edited on July 4, 2005, 6:55 am

Posted by K Sengupta on 2005-07-01 06:42:48