Define it!

Let n n be the number of ordered quadruples ( x 1 , x 2 , x 3 , x 4 ) (x_{1}, x_{2}, x_{3}, x_{4}) of positive odd integers that satisfy,

k = 1 4 x k = 98 \displaystyle \sum_{k=1}^{4}x_{k}=98

Find n 100 \dfrac{n}{100} .


The answer is 196.

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1 solution

Rishabh Jain
Feb 10, 2016

Since x 1 , x 2 , x 3 , x 4 x_1,x_2,x_3,x_4 are odd, let x 1 = 2 p + 1 , x 2 = 2 q + 1 , x 3 = 2 r + 1 , x 4 = 2 s + 1 x_1=\color{#0C6AC7}{2p+1},x_2=\color{#D61F06}{2q+1},x_3=\color{#456461}{2r+1},x_4=\color{#EC7300}{2s+1} for some integers p,q,r,s.
Substituting them in the given expression, given equation simplifies to: p + q + r + s = 47 \color{#0C6AC7}{p}+\color{#D61F06}{q}+\color{#456461}{r}+\color{#EC7300}{s}=47 From here it's a regular combinatorics problem asking for non negative integers p , q , r , s p,q,r,s satisfying the above equation, which are given by ( 47 + 4 1 47 ) = ( 50 47 ) \binom{47+4-1}{47}=\binom{50}{47} ( 50 47 ) 100 = 196 \Large\therefore \dfrac{\binom{50}{47}}{100}=\huge\color{#302B94}{\boxed{\color{#007fff}{196}}}

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