The random variable a obeys binomial distribution, and is denoted as a ~ b (n, p).

Where n is the number of tests, that is, 80 p is the probability of success, that is, the probability of throwing five coins at a time with two heads up and three heads down is the fifth power of the combination number c (5,2) * (1/2).

And the formula e = NP.

So e (a) = 25.