The probabilities of all possible outcomes in a distribution must sum to 1.

The domain of a function f(x) is the set of all values for which the function is defined, and the range of the function is the set of all values that f takes.
We know that the won 3/4. So that means that the remaining 1/4 is the games lost, which is 21.
To find the total games played, we simply multiply 21 by 4 to get 4/4, or the total games.