Poisson Distribution question

Can someone help me with part c?

The number of people arriving for treatment at an emergency room can be modeled by a Poisson process with a rate parameter of five per hour.

a) What is the probability that exactly four arrivals occur during a particular hour?

P(X=4)=$\displaystyle \frac{e^{-5}*5^{4}}{4!}$ = .17547

b) What is the probability that at least four people arrive during a particular hour?

1 - P(X=3) - P(X=2) - P(X=1) - P(X=0) = 1 - .14037 - .08422 - .03369 - .00674 = .73498

c) How many people do you expect to arrive during a 45-minute period?

$\displaystyle \lambda=\alpha t$, if $\displaystyle \alpha$=5/hr and t = 3/4 hr we get 15/4. Do we take the floor, round up if > .5, or take the ceiling? I can't find the answer in my notes, or in the text.