\[ P(X \leq k) = \sum_{k=0}^n \binom{n}{k} p^k (1-p)^{n-k} \]
n=5
n=10
n=15
n=20
n=25
\[ P(X = k) = \frac{e^{-\lambda}\lambda^k}{k!}, \quad k=0,1,2,\dots \]
\[ a)\lambda\leq 1\]
\[ b) 2\leq \lambda\leq 20\]