PRACTICE EXERCISES

1.  How many integers from 1 through 999 do not have any repeated digits?

2. In a certain state, license plates consist of from zero to three letters followed by from zero to four digits,with the provision,  however,
    that a blank plate is not allowed.
a. How many different licence plates can the state produce?
b. Suppose 85 letter combinations are not allowed because of their potential for giving offense. How many different license plates
    can the state produce?