Question 149737
would be interested in seeing the instructor's solution


there are 20C3 (1140) possible groups of 3 in 20 __ some of these groups contain non-purchasers


there are 18C3 (816) possible groups of 3 in 18 __ none of these groups contain non-purchasers


probability of all purchasers is 816/1140 or 0.716 (approx)


if order of selection is thought to be important (permutation vs combination); the result will be the same