There is a betting where you bet on football teams. There are three options available for every game played, 1, X or 2. 1 = win for home team, X = tie and 2 = win for visiting team.
On every seies you bet on 13 teams: 1, X or 2.
How many different series are there with exactly
a) 11 "rights"
b) n "rights", where 0<= n<=13