Here, we have to find how many 3-digit numbers can be formed without using the digits 0, 2, 3, 4, 5 and 6. i.e we have to find how many 3-digit numbers can be formed using the digits 1, 7, 8, 9. Clearly, repetition of digits is allowed. The number of ways to fill unit's digit = 4 The number of ways to fill ten's digit = 4 The number of ways to fill hundred's digit = 4 ∴ Total number of required numbers = 4 × 4 × 4 = 64