help answering these questions for Discrete Struct class. I can not press show answer. doesnt work.

For questions 2 and 3, express your answer as 'n choose r. Ex: 8 choose 3. 1) Define a bijection between 5-subsets of the set S (1,2,3,4, 5,6,7,8) and 8-bit strings with exactly five 1's. A subset X of S with five elements maps on to a string x so that j E X if and only if the jth bit of x is 1. What string corresponds to the set (1,3,4, 5, 8)? Check Show answer 2) How many 8-bits strings have exactly five 1's. Check Show answer 3) How many strings over the alphabet (a, b) have length 20 and exactly 8 a's? Check Show