How to find the frequency of a particular element of a sequence( with distinct elements) of e elements in its all subsequences of a particular length say l.
I tried to proceed using permutation and combination , but it is giving wrong answer.
Any better method to solve this?
A= Total number of subsequence of length l = $ e C l $
B= Total number of subsequence of length l without particular element = $(e-1) C l$
So required result is A - B = $ (e-1)C(l-1) $