A four or five$-$digit PIN is to be selected from the nine digits $0,1,2,3,4,5,6,7,8.$

How many $4-$digit PINs are possible if:

a$)$ all $4$ digits are the same? e$.$g$.3-3-3-3$

b$)$ no digit can be repeated?

c$)$ any digit can be repeated?

d$)$ the first $2$ digits must be different digits and the last $2$ digits must be the same, but none of the first $2$ digits can be used in the last $2?$

Suppose any digit can be repeated, how many $4-$digit PINs are possible if each PIN:

a$)$ must start with $"7"?$

b$)$ must not end with $"0"?$

c$)$ must neither start with $"5"$ nor end with $"5"?$

Suppose no digit can be repeated, how many $5-$digit PINs are possible if each PIN:

a$)$ must start with $"7"?$

b$)$ must not end with $"0"?$

c$)$ must neither start with $"5"$ nor end with $"5"?$