Let's arrange some balls

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 N N . Find the sum of digits of N N .

Note: The sum of digits of 351 351 is 3 + 5 + 1 = 9 3+5+1=9 .


The answer is 45.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Mark Hennings
Nov 30, 2016

The number N N of ways a score of 40 40 % can be achieved is equal to the coefficient of x 240 x^{240} in the expression ( 1 + x + x 2 + + x 100 ) 6 = ( 1 x 101 1 x ) 6 = ( 1 x 101 ) 6 ( 1 x ) 6 = j = 0 6 ( 1 ) j ( 6 j ) x 101 j × k = 0 ( k + 5 5 ) x k (1 + x + x^2 + \cdots + x^{100})^6 \; = \; \left(\frac{1-x^{101}}{1-x}\right)^6 \; = \; (1 - x^{101})^6(1-x)^{-6} \; = \; \sum_{j=0}^6 (-1)^j{6 \choose j}x^{101j} \times \sum_{k=0}^\infty {k+5 \choose 5}x^k (for x < 1 |x| < 1 ), and hence N = j = 0 2 ( 1 ) j ( 6 j ) ( 245 101 j 5 ) = 4188528351 N \; = \; \sum_{j=0}^2 (-1)^j {6 \choose j}{245-101j \choose 5} \; = \; 4188528351 Thus the digit sum of N N is 45 \boxed{45} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...