BdMO National 2013: Junior 6

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BdMO
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BdMO National 2013: Junior 6

Unread post by BdMO » Fri Jan 10, 2014 1:25 am

You have $1, 2, 3, 4, 5, 6, 7, 8, 9$ kg weights in your home. You have only one piece of each of the weights. You also have a balance. If you put a weight on the left side of that balance, its weight becomes twice. Every time you choose three weights to put on the balance in such a manner that the balance remains in equilibrium. On every turn, the total weight the balance can carry gets reduced by $3$ kg. At the first time the balance may carry at most $20$ kg in total. In how many ways can you keep weights on balance?

Kiriti
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Re: BdMO National 2013: Junior 6

Unread post by Kiriti » Fri Jan 10, 2014 11:47 am

এইটার উত্তর mathematically কিভাবে বের করবো ?? গুনে গুনে না ।
"Education is the most powerful weapon which you can use to change the world"- Nelson Mandela

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Labib
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Re: BdMO National 2013: Junior 6

Unread post by Labib » Fri Jan 10, 2014 10:19 pm

If you are looking for some hints,
If you think logically, you will realize that you do not need to use brute force for this problem. (In fact, you cannot. There are $2*9*7*4=504$ choices for the 3 weights.)
case 1: You put 1 weight on the left.
Let, $w_L, w_{R_1}$ and $w_{R_2}$ be the three weights you chose at first. Here, $w_L$ is the weight you put on the left.
According to the statement,
$2w_L = w_{R_1} + w_{R_2}$
and,
$w_L + w_{R_1} + w_{R_2} \leq 20$
$\Rightarrow w_L \leq 6$

case 2: You put 2 weight on the left.
Let, $w_R, w_{L_1}$ and $w_{L_2}$ be the three weights you chose at first. Here, $w_R$ is the weight you put on the right.
$2(w_{L_1} + w_{L_2}) = w_R$
and,
$w_R + w_{L_1} + w_{L_2} \leq 20$
$\Rightarrow w_{L_1} + w_{L_2} \leq 6$

Use these conditions to your favour. Think about the possible values of the next $w_L$ and $w_R$.
*edited to add a missed out case. (thanks to Mahi)
Last edited by Labib on Sat Jan 11, 2014 10:53 pm, edited 1 time in total.
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*Mahi*
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Re: BdMO National 2013: Junior 6

Unread post by *Mahi* » Sat Jan 11, 2014 7:09 pm

Labib wrote: According to the statement,
\[2w_L = w_{R_1} + w_{R_2}\]
The question allows $2(w_{L_1}+w_{L_2}) = w_R$ too. Did you count those?
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Labib
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Re: BdMO National 2013: Junior 6

Unread post by Labib » Sat Jan 11, 2014 10:35 pm

*Mahi* wrote:
Labib wrote: According to the statement,
\[2w_L = w_{R_1} + w_{R_2}\]
The question allows $2(w_{L_1}+w_{L_2}) = w_R$ too. Did you count those?
Missed out on those I guess. :? :?
I'll fix it anyway.
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atiab jobayer
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Re: BdMO National 2013: Junior 6

Unread post by atiab jobayer » Thu Jan 30, 2014 11:36 pm

Vai Eku sohoj basai lekhen
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Labib
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Re: BdMO National 2013: Junior 6

Unread post by Labib » Fri Jan 31, 2014 2:31 pm

I'm not sure if I can make it sound any easier than this Atiab. :-(
I'd leave it to someone else who could come up with an easier solution.
Meanwhile, I'd give you a heads up. Please do not use "Banglish" in the forum. (I believe it is prohibited)
If you wish to right bangla, please use Avro/some other software.
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atiab jobayer
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Re: BdMO National 2013: Junior 6

Unread post by atiab jobayer » Fri Jan 31, 2014 2:55 pm

I want to know what is the final answer
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asif e elahi
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Re: BdMO National 2013: Junior 6

Unread post by asif e elahi » Fri Jan 31, 2014 11:51 pm

I think the answer is 1.I solved it by brute force.

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Re: BdMO National 2013: Junior 6

Unread post by Fatin Farhan » Fri Jan 31, 2014 11:57 pm

আমারও ১ আসছিল। গুনে গুনে করসিলাম :?
"The box said 'Requires Windows XP or better'. So I installed L$$i$$nux...:p"

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