There are 10 digits from 0 to 9 When you say 4 digit number that means it can't start with zero, then 1) there are 9 ways to chose the 1st digit (excluding zero) 2) there are only 1 ways to chose the 2nd digit (only 8)) 3) there are 10 ways to chose the 3rd digit 4) there are 10 ways to chose the 4th digit
