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

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Q 2473001846.     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?
IAS 2016 Prelims
A

23 minutes

B

22 minutes

C

21 minutes

D

20 minutes

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