+9665 First we figure out what's (10^27 - 1800) mod 9
(10^27 - 1800) mod 9
= 10^27 mod 9 - 1800 mod 9
= 10^27 mod 9
Then we find sequence for 10^n mod 9
10^1 mod 9 = 1
10^2 mod 9 = 1
10^3 mod 9 = 1
10^4 mod 9 = 1
Therefore 10^27 mod 9 = 1
Substitute (10^27 - 1800) mod 9 = 1 into the eq.
1 - u ≡ 1 mod 9
u = 0
