Test Index

UPSC Civil Services 2016 Prelims General Studies Paper II

© examsnet.com

Question : 79 of 80

Marks: +1, -0

Four friends A , B , C and D need to cross a bridge. A maximum of two persons can cross it at a time. It is night and they just have one lamp. Persons that cross the bridge must carry the lamp to find the way. A pair must walk together at the speed of slower person. After crossing the bridge, the person having faster speed in the pair will return with the lamp each time to accompany another person in the group. Finally, the lamp has to be returned at the original place and the person who returns the lamp has to cross the bridge again without lamp. To cross the bridge, the time taken by them is as follows:

A : 1 minute, B : 2 minutes, C: 7 minutes and D : 10 minutes. What is the total minimum time required by all the friends to cross the bridge?

23 minutes
22 minutes
21 minutes
20 minutes

Solution:

Here, A takes the shortest time to cross the bridgei.e. 1 min. And,

D

takes

10\,\text{min},\,C

takes

7\,\text{min}

and

B

takes

2\,\text{min}

to cross the bridge.

So, 4 friends can cross the bridge in minimum time in the following ways.

1.

A+B

crossing and A returning

:(2+1)\,\text{min}

2.

A+C

crossing and A returning :

(7+1)\,\text{min}

3.

A+D

crossing and A returning

:(10+1)\,\text{min}

4. A crossing the bridge for the last time: 1 min.

Hence, total minimum time

=3+8+11+1=23

\text{min}

© examsnet.com

Go to Question:

Prev Question

Next Question