The meaning of XY is obvious, that is, the value of x times the value of y, and y can only be 0 when x is 2. (An experiment is always * * * done twice, and A 1 appears twice, so the number of A2 appearances is naturally zero. ) Similarly, when y is 2, x can only be 0. When x is 1, y may be 1 or 0 (because of event A3). So the values of XY are only 0 and 1.