Question

which digit must replace? to make the number 30?4divisible by 9 ?

Answer 1

The digit 4 should be replaced by 6 in 304 to make it divisible

306 / 9 = 34

Answer 2

Assuming that it's a typo error and the given is 304

To know if a number is divisible by 9, the sum of its digits must be divisible by
3 + 0 + 4 = 7
Yep, 304 is not divisible by 9. We could replace 4 to 6
3 + 0 + 6 = 9
So obviously, it is divisible by 9. You could try to manually dividing it but simply knowing the rules is better. ^_~