Passwords on a certain system have exactly 5 letters that ar

Passwords on a certain system have exactly 5 letters that are either lowercase
letters or uppercase letters.
(a) How many possible passwords are there?
(b) How many possible passwords are there that use only lowercase letters?
(c) How many possible passwords are there that use only uppercase letters?
(d) How many possible passwords are there that use at least one uppercase
letter and at least one lowercase letter?

Solution

a. 2. (26^5)=23762752

b.26^5=11881376

c. 26^5=11881376