Question

A. Seventy-five percent of adult smokers started smoking before turning 18 years old. If 20 adult

smokers are randomly chosen, find the probability that at most 6 of them started smoking after they

turned 18. Round answer to 3 significant digits .

B. A multiple choice quiz has 12 questions. Each question has four possible answers. Assume that a

student randomly guesses on each question. What would it be unusual for the student to guess

correctly on 7 questions? Use the mean and standard deviation in order to answer this question.

Answer 1

A.

given data:

probability for smokers smoke started before turning 18 year old, x=0.75

probability for smokers smoke started after turning 18 year old, p=1-x=0.25

number of smokers, n=20

probability that at most 6 of them started smoking after they turned 18. P(6,X>18)

r	0	1	2	3	4	5	6
Probability	0.003171	0.021141	0.066948	0.13389562	0.189685	0.20233115	0.168609

P(6,X>18) is sum of probability

r	0	1	2	3	4	5	6	Total
probability	0.003171	0.021141	0.066948	0.13389562	0.189685	0.20233115	0.168609	0.785782

the probability that at most 6 of them started smoking after they turned 18 is 0.785782