Assume that the prob. of at least one typo in the slides is 0.2.
What is the prob. that you find at least 2 typos?
I realize that typos are a rare event, so I need to use a Poisson Approximation to find this probability. Let $X\sim\text{Poisson}(\lambda)$. The relevant formula is $$\mathbb{P}(X=k)=e^{-\lambda}\frac{\lambda^k}{k!}$$ From the problem, I am given $\mathbb{P}(X\geq1)=0.2$. and the probability I am trying to find is $\mathbb{P}(X\geq 2)$. I think that it is easier to find $\mathbb{P}(X<2)$ so I will take its complement $\mathbb{P}(X\geq2)=1-\mathbb{P}(X<2)$. Then, my desired probability is found as follows: $$\mathbb{P}(X\geq2)=1-\mathbb{P}(X=1)$$ My problem is that I do not know how to find $\lambda$ from the given information.
$$\mathbb{P}(X\geq 1)=1-\mathbb{P}(X=0)=1-e^{-\lambda}=0.2$$
thus easy find $\lambda=-\log 0.8\approx 0.223$