Question

Postini is a global company specializing in communications security. The company has

determined that 91% of all e-mails are actually spam. If you check your e-mail in the morning
and see that you have 25 new messages, what is the probability that:
a. (5 points) Exactly 5 of them are spam?
b. (5 points) Less than 20 of them are spam?
c. (5 points) Ten or more of them are spam?

Answer 1

a. The probability that exactly 5 of the 25 emails are spam is approximately 0.228, or 22.8%. This can be calculated using the binomial distribution formula: P(X = 5) = (25 choose 5) * (0.91)^5 * (0.09)^20, where "choose" is the binomial coefficient.

b. The probability that less than 20 of the 25 emails are spam is 0000….