If a PIN number consists of 6 characters and each of these characters could be one of the 26 alphabets or a number from 1 to 9, then how many different PINS are there where:
a) all five characters are the same and one character is different.
b) four characters are the same and two characters are same but different from the other four.
c) four characters are the same and two are different(i.e different from the other four and different from each other).
d) How many PINs are reverse of another PIN.(i.e reverse of ABCD12 is 21DCBA).
Can anyone explain how to approach these questions? I'm new to the topic.