+673 When $N$ is divided by $10$, the remainder is $a$. When $N$ is divided by $2$, the remainder is $b$. What is $N$ modulo $20$, in terms of $a$ and $b$?
+16 N mod 10 = a
N mod 2 = b
N mod 20 =?
2 and 10 are not coprime. 10 is multiple of 2.
If a is odd then b=1.
If a is even then b=0.
We don’t need b.
N mod 20 ={a, a+10}
