$x \equiv 4 \pmod{9}$. What is $5x \bmod 9$?

Explanation

1. 01

   A congruence can be multiplied through: $5x \equiv 5 \times 4 = 20 \pmod{9}$.

2. 02

   $20 = 9 \times 2 + 2$, so $5x \bmod 9 = 2$.