$X$ follows a Poisson distribution with $\lambda = 1.5$. What is $P(X \le 1)$? (Take $e^{-1.5} = 0.2231$.)

Explanation

1. 01

   $P(X \le 1) = P(0) + P(1) = e^{-1.5}(1 + 1.5)$.

2. 02

   $= 0.2231 \times 2.5 = 0.55775$.