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:
[tex] 26^{6}=308915776 [/tex]
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:
[tex] 26^{6}=308915776 [/tex]
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:
[tex] 52^{6}=19770609664 [/tex]
