Hello, I'm new member, I found this website is very helpful. I need help with 2 review problems from my discrete class, please help me, thank you.

1) Use inclusion-exclusion to find the number of 4-digit numbers with at least one 8 ( hint: let A_j = the set of all 4-digit numbers with an 8 in position i).

here is my answer : 9 x 10 x10 x10 - 8 x 9 x 9 x 9 = 3168 , I'm not sure i get it right.

2) Find the number of ways each situation below can occur:

a) A ticket office has 5 reserved seat tickets and 8 general admission tickets to sell to 15 customers.

b) Billy gets to pick 9 candy bars from the candy rack, which has Snickers,Mars,Payday, Mounds, and Crunch.

I don't know how to do part a. For part b, I got P(9,5) = 15120