One out of a thousand bulbs manufactured by a factory is found to be defective. If 2,000 bulbs were produced, find the probability that at least two bulbs are defective.
Worked solution (try it first)
n=2000 is large and p=0.001 is small, so use the Poisson approximation with λ=np=2.
"At least two" is 1 minus the chances of 0 and 1: P(X≥2)=1−e−2(1+2)