If half the batch of 6,000 vials are contaminated, what is the probability of finding no contaminated vials if only two vials are selected from the total amount produced?
Use the binomial distribution.
If you calculate by hand, here is a formula:
P(x) = (nCx)(p^x)[q^(n-x)]
x = 0
n = 2
p = .5
q = 1 - p = .5
Substitute into the formula and calculate.
I hope this will help.