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Mumsnet Discussions: Education : Maths homework (11 messages)
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Contact the poster Contact mumsnet about this post By Beetroot on Sun 27-Apr-08 18:07:50
4 friends need to cross a bridge
they start on the same side of the bridge a maximum of two people can cross at anytime. It is night and they have jsut one lamp. People that cross the bridge must carry thelamp to see the way.
A pair must walk together at the rate of the slower person
Rachel takes one minute to cross
Ben takes two minutes to cross
george takes 7 minutes to cross
and Yvonne takes 10 minutes to cross

The fastest time they all get across takes 17 minutes

Can you work out how it is done?
Contact the poster Contact mumsnet about this post By Beetroot on Sun 27-Apr-08 18:08:17
baffling all of us here
Contact the poster Contact mumsnet about this post By Beetroot on Sun 27-Apr-08 18:13:42
?
Contact the poster Contact mumsnet about this post By controlfreakyagain on Sun 27-Apr-08 18:14:41
fastest time i can get is 21 mins.... r + b = 2mins... then r 1 min back.... then r + g =7 mins then r 1 min back then r + y = 10 mins = 21 mins

but then i'm pretty crap at maths? what year is dc in?
Contact the poster Contact mumsnet about this post By clam on Sun 27-Apr-08 18:14:56
Yes, I know this one...... but can't remember off-hand grin. Got a feeling a couple have to go together and return, and maybe one goes halfway and then comes back........? Will ask DH. He'll know. Hang on
Contact the poster See this person's profile Contact mumsnet about this post By avenanap on Sun 27-Apr-08 18:18:02
Hmm. ds says to throw the lamp back.

If Georga end Yvonne go together, that takes 10 mins, George throws lamp back, Rachel and Ben go together, taking 2 minutes. It takes them 12 minutes in total.
Contact the poster Contact mumsnet about this post By PuppyDogTails on Sun 27-Apr-08 18:18:20
We can't see how it works.

It takes 17 minutes to get George and Yvonne across. Even if they go together then George still has to come back which brings you to 17 minutes.
Contact the poster Contact mumsnet about this post By NamelessNewbie on Sun 27-Apr-08 18:20:07
R & B cross, 2mins
R returns, 1 min
G & Y cross 10 mins
B returns 2 mins
R & B cross 2 mins
Contact the poster Contact mumsnet about this post By controlfreakyagain on Sun 27-Apr-08 18:21:33
oh well done nn! it's obvious to me NOW!
Contact the poster Contact mumsnet about this post By PuppyDogTails on Sun 27-Apr-08 18:21:49
Aaaaaaah!
Contact the poster Contact mumsnet about this post By Beetroot on Sun 27-Apr-08 18:26:42
oh yes - oh yes - very good


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