This is the question:

Every box of a smartphone has a coupon with a number inside. the number can be: 1,2,3,4,5. If someone get all the coupons with all the numbers, he win a free phone.

the first question is: "what is the probability of a person who bought 5 phones, NOT to win a free phone". and the answer is:0.9616. - this part wasn't the problem.

The following question, is the one I cannot solve:

"What is the probability of a person who bought 8 phones, to win a free phone"

the problem I have is that now he is buying 8 phones, and I'm not sure what I need to do...

Any help would be greatly appreciated.