Monday Morning Braintease (the original)
Four bankers, Al, Bob, Cal and Dave need to get across a river. The only way to cross the river is by an old bridge, which holds at most 2 people at a time. Being dark, they can't cross the bridge without a flashlight, of which they only have one. So each pair can only walk at the speed of the slower person. They need to get all of them across to the other side as quickly as possible. Al is the slowest and takes 12 minutes to cross; Bob takes 5 minutes; Cal takes 3 minutes; and Dave takes 1 minute.
What is the minimum time to get all of them across to the other side?
Show thought process for SBs.
22 minutes. Dave is the fastest so he crosses with each banker and returns on his own to get the other bankers. Total time is 12+5+3 plus two 1 minute return trips.
Al and Dave go first across the bridge, this takes 12 minutes. Since Dave is the fastest, he goes back alone with the flash light, +1 minute Dave and Bob go across the bridge, +5 minutes Dave goes back alone with the flash light, +1 minute Dave and cal cross the bridge with the flashlight, +3 minutes
Total= 22 minutes
I got that really quickly so I'm curious to know where I fucked up.
Can anyone do it faster?
Al and Bob are liabilities so group together. Cannot be first pair - need to transport light back.
Send Cal and Dave over first. total = 3 mins Send Dave back. total = 3 mins + 1 min = 4 mins Al and Bob go over together. total = 12 mins + 4 mins = 16 mins Cal comes back. total = 16 mins + 3 mins = 19 mins Cal and Dave cross together. total = 3 mins + 19 mins = 22 mins
Total Time: 22 mins
Edit: oh crap i wanted to be the first to post. haha
Is 22 the right answer? I mean that was straightforward if so
Yea it is right
correct. need to find a more challenging brainteaser. maybe post this afternoon.
Nah i can do this faster.
Dave crosses with Cal = 3 minutes Dave walks back = 1 minute Total = 4 minutes Dave hands flashlight to Bob who crosses with fatass Al = 12 minutes Total = 16 minutes Bob hands flashlight to Cal who crosses to get dave = 3 minutes Total = 19 minutes Poor Cal has to walk back again with Dave again = 3 minutes Total = 22 minutes FUCK I THOUGHT I HAD IT
edit: nevermind, i didn't see that this method was already tried
Just FYI for OGD and Lekman - read H&D's answer for the "correct" solution to this problem. It happens that in this case, it is no faster than the "obvious" solution, but if the numbers are tweaked it might be a faster method to get everyone across.
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