'Four friends A B C D need to cross bridge' Whatsapp puzzle & answer with solution inside

The Coronavirus pandemic has held the world in its clasp. The number of people getting affected by it has been increasing exponentially despite the world being in lockdown and people being forced to stay indoors. In this tough time, people have resorted to social media to keep in touch with their friends and family. There has been a rising trend among the people to send each other puzzles and quizzes to keep each other entertained and share laughter during these tough times. One such puzzle that has been doing the rounds on social media is a series of riddles about 4 friends who need to cross a bridge. Given below is the question along with its answer and explanation.

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Question

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 a 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 a 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?

(a) 23 minutes 

(b) 22 minutes 

(c) 21 minutes  

(d) 20 minutes

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Answer: (a) 23 minutes

Solution

Here, A takes the shortest time to cross the bridge i.e. 1 min. And, D takes 10 min, C takes 7 min and B takes 2 min to cross the bridge.

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

 A + B crossing and A returning : (2 + 1) min 2. A + C crossing and A returning : (7 + 1) min 3.

A + D crossing and A returning : (10 + 1) min 4.

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

Hence, total minimum time = 3 + 8 + 11 + 1 = 23 min.

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