Sequence combinatory

[Guest]

I'm struggling with this problem can you guys help me?
Let S be the set of all sequences (a, b, c, d, e) with expressions belonging to the set of numbers {0.1,..., 9}. How many sequences are there in the set S
a) decreasing?
b) whose product abcde is an even number?
c) in which the sum of the digits of the product abcde in decimal notation is divisible by 9?

Thanks for responding