1258 isn't divisible by 9. 1251 and 1260 are. The digits of numbers that are divisible by 9 add up to a multiple of 9. If you were to stick a 2 in there somewhere, the result would be divisible by 9.
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139.7778
2 or 8
8
It needs an 8.
81: 8 + 1 = 9 which is divisible by 9, so 81 is divisible by 9 162: 1 + 6 + 2 = 9 which is divisible by 9, so 162 is divisible by 9 199: 1 + 9 + 9 = 19 → 1 + 9 = 10 → 1 + 0 = 1 which is not divisible by 9, so 199 is not divisible by 9. 1125: 1 + 1 + 2 + 5 = 9 which is divisible by 9, so 1125 is divisible by 9. So 199 is the only one not divisible by 9.