Let N represent the number of ordered pairs of positive integers ( x 1 , x 2 , . . . , x 2 0 1 ) such that ∑ i = 1 2 0 1 x i = 2 0 1 4 , and, for some k ∈ { 1 , 2 , . . . , 2 0 1 } , x k ≥ 1 4 . If N = ( m 1 0 m ) , for some positive integer m , find the hundreds digit of m .
Notes:
∑ n = 1 k x n = x 1 + x 2 + . . . + x k .
( k n ) = n C k (combination).
This problem is part of the set Yay for 2014! .
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Let the some x k = x k ′ + 1 3 .
Then we just need the number of ways for x 1 + x 2 + . . x k ′ + . . x 2 0 1 = 2 0 0 1 where 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 .