Dark bridge

Level 2

Aaron, Betsy, Charlie and David are crossing a dark bridge. It is night so they need a flashlight to get across. They only have 1 flashlight and they can't throw it across. They all walk at different speeds. Only 2 people can cross at a time and they have to walk at the speed of the slowest one. What is the shortest amount of time they can take so all of them will get across? The speeds are

Aaron can cross in 1 minute

Betsy can cross in 2 minutes

Charlie can cross in 5 minutes

David can cross in 10 minutes


The answer is 17.

Puzzlefizzy Is awesome by Puzzlefizzy Is awesome , 20, USA

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2 solutions

It takes 17 minutes by

Aaron and Betsy -> 2 mins Betsy <- 4 mins Charlie and David -> 14 mins Aaron <- 15 mins Aaron and Betsy -> 17 mins

3 Helpful 0 Interesting 0 Brilliant 0 Confused

Aaron and betsy = 2mins, aaron goes back = 1 min, aaron and charlie 5 mins, aaron goes back = 1 min= aaron and david = 10 mins 2+1+5+1+10=19

Cisco Wicaksana - 6 years ago

Why not: First- David and Charlie=> 10 minutes, Second- Betsy and Aaron=> 2 minutes, for Total = 12 minutes

Sai Sam - 6 years, 1 month ago

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because they cant throw the flashlight

Cisco Wicaksana - 6 years ago

They only have the one flashlight. Consider the return trip for one person. Now that person has to come back with another person (only 2 at a time). One person still remains on the wrong side and must have one of the previous crossers come back for them too.

George Dodd - 4 years, 7 months ago
Saya Suka
Apr 26, 2021

Answer
= ceiling[1,2] + 1 + ceiling[10,5] + 2 + ceiling[1,2]
= 2 + 1 + 10 + 2 + 2
= 17 minutes


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