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#1 2012-10-05 22:54:21

anna_gg
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House painting

3 friends, Alan, Brian and Chester, paint a house. If Alan had to paint it on his own, it would take him one hour more than the time it would take for all three to paint it together. If Brian  had to paint it on his own, it would take him five hours more than the time it would take for all three to paint it together, and Chester 8 hours more.
How much time would it take for Alan and Brian to paint it together?

#2 2012-10-06 03:47:59

anonimnystefy
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Re: House painting

I got


The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“Taking a new step, uttering a new word, is what people fear most.” ― Fyodor Dostoyevsky, Crime and Punishment

#3 2012-10-06 07:39:17

bobbym
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Re: House painting

Hi anna_gg;


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

#4 2012-10-06 22:29:41

anna_gg
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Re: House painting

Let A, B and C be the hours needed for each friend to paint the house on his own and D the number of hours needed if they paint all together.
Then
A = D+1
B = D+5
C = D+8

In one hour, Alan has painted 1/A of the house, Brian 1/B and Chester 1/C, so all together have painted 1/A+1/B+1/C.
Thus in D hours they have painted D*(1/A+1/B+1/C) and we have:
D*(1/A+1/B+1/C)=1 (the entire house).
By substituting A, B and C from the above equations, we finally get:

D^3 + 7D^2-20=0. We keep the only acceptable solution D=1,531129 (the other two are negative).
So we have: A=2,531129  B=6,531129
Now that A and B will paint the house:

Let X be the number of hours we are looking for.

Χ*(1/A+1/B)=1. By substituting the above values of A and B, we get X = 1,824173, which is what you got, guys!

Thanks!!

Last edited by anna_gg (2012-10-06 22:30:20)

#5 2012-10-06 22:32:50

bobbym
Administrator

Online

Re: House painting

Hi;

Do you know if we got the right answer?


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

#6 2012-10-07 00:27:15

anna_gg
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Re: House painting

Yes!

#7 2012-10-07 07:30:20

bobbym
Administrator

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Re: House painting

Hi anna_g;

That is good to hear. Thanks for the problem.


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

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