Counting passwords
A password should be six to eight characters long, where each
character is either an uppercase letter or a digit. Each password should
contain at least one digit. How many passwords are there?
P6 - the number of 6-character passwords
P7 - the number of 7-character passwords
P8 - the number of 8-character passwords
the number of all passwords = P6+P7+P8 = 2,684,483,063,360