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Among the total 9 boxes, you have 2 boxes with prizes and 9-2 = 7 regular boxes without prizes.
(a) In this case, the buyers actually selected 6 regular boxes from the set of 7 regular boxes.
They can do it in = 7 ways.
This set is the set of favorable events in this case.
The total space of events has = = 84 elements.
Therefore, the probability in this case is = . ANSWER
(b) This probability is the complement to the probability of case (a):
P = 1 - = . ANSWER
(c) We calculate the number of favorable events in this case as follows:
- you can choose 2 prize boxes from 3 prize boxes by 3 ways, and
- you can choose the complementary 6-2 = 4 regular boxes from 9-3 = 6 regular boxes by = = 15 ways.
- so the number of favorable events is 3*15 = 45,
- while the number of all elements of the event space is = 84.
So the probability in this case is = . ANSWER
All questions are answered : the problem is solved.