A committee of five is to be chosen from
a group of 9 people. The probability
that a certain married couple will either
serve together or not at all is
Please in an arithmetic mean
a, A_1 , A_2 , A_3 , ... , A_n , b
where A_1 , A_2 , A_3 , ... , A_n are nth arithmetic mean
why is b = (n + 2)th term: like T_(n + 2)
Please