How could you find the number of different arrangements of 6 letters in a computer password?

Solution: If we are allowed to use only lower case letters, then we have 26 letters available with us. From these 26 letters the number of different arrangements of 6 letters is:

Now if we are allowed to use only upper case letters, then we have 26 letters available with us. From these 26 letters the number of different arrangements of 6 letters is:

if we are allowed to use both  upper case and lower case letters, then we have 52 letters available with us. From these 52 letters the number of different arrangements of 6 letters is: