Originally posted on 08/28/2016:

Easiest way is take the number of plays and subtract 1 since you can't parlay a team with itself. Take that number and divide by 2. Take that number and multiply it with the total play number.

so your example would be:

27-1 = 26/2 = 13 and 13*27 = 351

11 team example:

11-1 = 10/2 =5 and 5*11 = 55