9
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Highest 2-digit number = 99 Highest 1-digit number = 9 Highest possible sum from 2, 2-digit numbers = 198 Highest possible sum from 2, 1-digit numbers = 18
987654321 is the highest nine-digit number with no repeated digits.
81 is.
6 is the only single digit perfect number.
There is no single digit number greater than 9. After 9 is 10, which is a double-digit number.