Using digits 0, 1, 2 and 3, how many numbers greater than 1000 can be made (repetition of digits is not allowed)?

Numbers greater than 1000 means 4-digit numbers (since with these 4 digits we can only form up to 4-digit numbers without repetition). First digit can't be 0: 3 choices (1, 2, or 3). Remaining 3 places filled with the other 3 digits: 3! = 6 ways. Total = 3 × 6 = 18.