Question

it has been reported that 40% of all internet users have been attacked by viruses. If 20 people are randomly surveyed, find the following probabilities:

a. At least 3 have been attacked
b. No more than 5 have been attacked
c. less than 15 have been attacked

Answer 1

The easiest way to do this problem is to use a binomial distribution table.

For a), n = 20, p = .4, x = 0, 1, 2
Add together the probabilities you find in the table, then subtract from 1 for your answer.

For b), n = 20, p = .4, x = 0, 1, 2, 3, 4, 5
Add together the probabilities you find in the table. This will be your answer.

For c), n = 20, p = .4, x = 15, 16, 17, 18, 19, 20
Add together the probabilities you find in the table, then subtract from 1 for your answer.

I hope this helps.