$P(X = x) = kx$ for $x = 1, 2, 3, 4$, and 0 otherwise. What is $P(X \ge 3)$?

Explanation

1. 01

   $k(1 + 2 + 3 + 4) = 10k = 1$, so $k = \frac{1}{10}$.

2. 02

   $P(X \ge 3) = P(3) + P(4) = \frac{3}{10} + \frac{4}{10} = \frac{7}{10}$.