$X$ takes the values 0, 1, 2 and 3 with probabilities 0.1, 0.3, 0.4 and 0.2. What is $P(X \le 2)$?

Explanation

1. 01

   $P(X \le 2) = P(0) + P(1) + P(2)$.

2. 02

   $0.1 + 0.3 + 0.4 = 0.8$, or $1 - P(3) = 1 - 0.2$.