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put this solution on YOUR website!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
