UPSC NDA Math Model Paper 9

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