According to a Harris Interactive poll conducted in 2009, 20% of American adults smoke. In a random sample of 500 American adults, what is the probability that no more than 100 smoke?

Answer 1

Find mean and standard deviation.

mean = np = (500)(.20) = ?
standard deviation = √npq = √(500)(.20)(.80) = ?

Note: q = 1 - p

Once you have calculated the mean and standard deviation above, use z-scores to find the probability:

z = (x - mean)/sd

Use 100 for x.

After you find the z-score, check a z-table to determine your probability.

I hope this will help get you started.