Potato chips are on sale, buy 2 get 1 free. There are only 9 bags of chips left (4 plain and 5 bbq). A shopper randomly grabs 3 bags. What is the probability the shopper will grab exactly 3 bags of bbq chips? Answers: A) 2 / 3 B) 1 / 3 C) 1 / 5 D) 5 / 42 E) 7 / 42

Rationale: combination probability formula : ncr = n ! / [ r ! ( n - r ) ! ] total possible , select 3 bags from 9 bags = 9 c 3 = 9 ! / [ 3 ! ( 9 - 3 ) ! ] = 84 . to have 3 bbq there must be 0 plain , select 0 plain from 4 = 4 c 0 = 1 . and , select 3 bbq from 5 = 5 c 3 = 10 . 3 bbq and 0 plain = ( 1 ) ( 10 ) = 10 probability = ( number outcomes favorable ) / ( total number outcomes ) = 10 / 84 = 5 / 42 answer : d
Answer: D