let if a student fails in n subjects where n <=9.then he must pass in atlest n+1 subjects.
so, tolal no. of successful ways= c(9,n+1) +c(9,n+2) + .......+c(9,8) + c(9,9). =s
so, toatal no. of unsuccesful ways=2^9 - s
=2^9-(2^9-∑c(9,n-k)) [where k is from 0 to n]
=∑c(9,n-k) ans.