Write all possible 2-digit numbers that can be formed by using the digits 3, 7 and 9. Repetition of digits is allowed.

We are required to write 2-digit numbers using the digits 3, 7 and 9, where repetition of digits is allowed.

Keeping 3 at ten's place, 2-digit numbers are 33, 37 and 39.

Keeping 7 at ten's place, 2-digit numbers are 73, 77 and 79.

Keeping 9 at ten's place, 2-digit numbers are 93, 97 and 99.

Hence, all possible 2-digit numbers are 33, 37, 39, 73, 77, 79, 93, 97 and 99.