A survey finds customers are charged incorrectly for an item 20​% of the time. Suppose a customer purchases 15 items. Find the probability that the customer is charged incorrectly on at most 2 items.

Answer 1

Pr(at most 2)= Pr(none) + Pr(1) + Pr (2)
this is a binomial probability ... (c + i)^15 ... sum the 1st three terms

(15C0 * .8^15) + (15C1 * .8^12 * .2 ) + (15C2 * .8^11 * .2^2)
using the Combination coefficient as nCx.