Answer:
It defines 1280 possible outcomes
Step-by-step explanation:
A can take on one of 20 possible values.Then, B can take on one of 64 possible values
The form to choose these pairs of outcomes are
(Possible values from A) x (Possible values from B) = 20 x 64 = 1280
Also the sum of these 1280 pairs of values must be equal to 1 because P(A,B) it is a joint distribution of probability.
We sum this 1280 pairs of values because A and B are discrete random variables.Otherwise we would use integrals.