Question 13
A 4-digit number is said to be ‘good’ if it uses exactly 3 different digits from the set {2, 0, 1, 7} (at most one of the digits used can be repeated). For example, 8712 and 7200 are said to be ‘good’ numbers, while 2017 and 7175 are not. How many ‘good’ numbers are there?