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# S2 Poisson Distribution question watch

1. The question is
'The number of breakdowns per day of the lifts in a large block of flats is modelled by a Poisson distribution with mean 0.2.
a) Find the probability that on a particular day there will be at least one breakdown.
b) Find the probability there are fewer than two days in a 30 day month with at least one breakdown.'
I managed part a fine and got the right answer (0.181 to 3 d.p.)
I'm stuck on part b though. I've tried using the binomial distribution formula because I thought there were a fixed number of trials (30) and a constant probability. I worked out the probability for one or zero days with at least one break down and added them but I got the wrong answer.....
I'm not sure what I'm doing wrong.
Thanks
2. What did you get as the answer to part (b) ?

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Updated: September 12, 2016
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