Alternatively, \( 78 \equiv 78 \mod 9 \): \( 7 + 8 = 15 \Rightarrow 1 + 5 = 6 \Rightarrow 78 \equiv 6 \pmod9 \), so \( 78^2 \equiv 6^2 = 36 \equiv 0 \pmod9 \). - Groen Casting