$Modulo\, remainder$

$999999999 = 10^9 - 1$

$10^9 = 2^9 × 5^9 = 2^5 × (2^4×5^9) = 32 × (2^4 × 5^9)$ ,

which means that $10^9$ is divisible by 32.

This also means, that since $10^9 - 1$ is 1 less than $10^9$ , its remainder should be less (than 0, or in this case, rather 32) by exactly 1 as well:

$32 - 1 = \boxed {31}$

999999999 = 12345678*81 + 81 apply modular arithmetic you get 31