If 30% of students over the age of 18 smoke and 9 students are chosen at random. What is the probability that less than 2 of them smoke.

Answer 1

p(s) = probability smoke , p(d) = probability don't smoke

p(s) + p(d) = .3 + .7 = 1

< 2 means one or none

(d + s)^9 = d^9 + 9 d^8 s + ... + 9 d s^8 + s^9

the sum of the 1st two terms is the solution
... (.7)^9 + [9 * (.7)^8 * .3]