Explanation:
the probability mass function of number of opponents in the game
We have Bernoulli trail ("success" = defeat, "Failure" = Victory)
P = 0.20
We are "waiting" for our first "success" (Our first defeat) q = 0.80
X is Geometric Random variable.
P(X = x) = pq^(x-1) = (0.20)×(0.80)^(x-1) ................X≥1