You seem to have part (a). Good work.
Not so good on part (b). First, "times" isn't a verb.
Second, you now have a binomial problem.
n = 20
The number of flaws on a magnetic tape produced continuously at a factory follows a Poisson distribution with an average of 0.01 falws per meter. A stand unit of this tape contains 250 meters of magnetic tape.
(a) What is the probability that there are at least two flaws in a single unit of tape?
(b) In a random sample of 20 units of tapes, what is the probability that at least two of them are flawless?
for part a I have 250x0.01=2.5 P(x>=2)=1-P(x=0)-P(x=1)=1-e^{-2.5}*2.5^0/0! -e^-2.5*2.5^1/1!
for part b, do I just times 2.5 by 20?
As previous posters have said, part (b) is a binomial question.
Step 1 Find probability that any given sample is flawless
The number of flaws in a sample of length 250 is . ie
It is flawless if X=0. You can find the probability using the formula for a poisson pmf:
This is the probability TKHunny gave you in post #2.
Step 2 Answer the question given
"You take 20 independent samples. Each is flawless with probability . Find the probability that at least 2 are flawless."
The number of flawed samples follows a binomial distribution.
Evaluate using the method you have been taught.