In a certain examination of 6 papers, each paper has 100 as maximum marks. Let the number of ways in which a candidate can secure 40% in the whole examination be . Find the sum of digits of .
Note: The sum of digits of is .
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The number N of ways a score of 4 0 % can be achieved is equal to the coefficient of x 2 4 0 in the expression ( 1 + x + x 2 + ⋯ + x 1 0 0 ) 6 = ( 1 − x 1 − x 1 0 1 ) 6 = ( 1 − x 1 0 1 ) 6 ( 1 − x ) − 6 = j = 0 ∑ 6 ( − 1 ) j ( j 6 ) x 1 0 1 j × k = 0 ∑ ∞ ( 5 k + 5 ) x k (for ∣ x ∣ < 1 ), and hence N = j = 0 ∑ 2 ( − 1 ) j ( j 6 ) ( 5 2 4 5 − 1 0 1 j ) = 4 1 8 8 5 2 8 3 5 1 Thus the digit sum of N is 4 5 .