Yay for 2014! #4

Let N N represent the number of ordered pairs of positive integers ( x 1 , x 2 , . . . , x 201 ) (x_{1}, x_{2}, ..., x_{201}) such that i = 1 201 x i = 2014 \sum_{i = 1}^{201} x_{i} = 2014 , and, for some k { 1 , 2 , . . . , 201 } k \in \{1, 2, ..., 201\} , x k 14 x_{k} \geq 14 . If N = ( 10 m m ) N = \binom{10m}{m} , for some positive integer m m , find the hundreds digit of m m .

Notes:

n = 1 k x n = x 1 + x 2 + . . . + x k \sum_{n = 1}^{k} x_{n} = x_{1} + x_{2} + ... + x_{k} .

( n k ) = n C k \binom{n}{k} = _{n}C_{k} (combination).

This problem is part of the set Yay for 2014! .


The answer is 2.

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1 solution

Alex Wang
Oct 31, 2014

Let the some x k = x k + 13. x_k=x_k'+13.

Then we just need the number of ways for x 1 + x 2 + . . x k + . . x 201 = 2001 x_1+x_2+..x_{k'}+..x_{201}=2001 where x i x_i is a positive integer greater or equal to 1. The number of ways for this (use stars and bars) is 2000C200 so m=200 and the hundreds digit is 2 \boxed{2} .

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