A car hire firm has three cars, which it hires out on a daily basis. Number of cars demanded per day follows a poisson distribution with mean 2.1
a) Find the probability that all cars are in use on any one day.
b) Find the probability that all cars are in use on exactly three days of a five day week.
What I tried:
For the a) part: normal poisson formula with mean = 2.1 and x = 3
Got the answer as 0.1890. The book's answer is 0.350
For the b) part: did poisson to binomial. Found out p = 0.7. Then used 5C3. But I think my method here is wrong.