Note:Sorry if this is considered a uni question, I'm not sure, so you'll have to tell me.

The question

A busy switchboard receives 150 calls an hour on average. Assume that the probability, $\displaystyle p_k$, of getting k calls in a given minute is:

$\displaystyle p_n = e^{-\lambda}\frac{\lambda^{k}}{k!}$

where $\displaystyle \lambda = s$ the average number of calls per minute.

a) Find the probability of getting exactly 3 calls in a given minute.

b) Find th probability of getting at least 2 calls in a given minute.

I'm not sure how to attempt this. I'm sure I'm missing something obvious, but there's no examples in this text to guide me. Any assistance would be great.