LCM( 39, 78 ) = 78
The formula for finding LCM is
$$ \text{LCM} (a, b) = \frac{ a \cdot b }{ \text{GCD}(a,b)} $$In this example we have: a = 39, b = 78 and GCD(39, 78) = 39.
$$ \text{LCM} (39, 78) = \frac{ 39 \cdot 78 }{ \text{GCD}(39,78)} = \frac{ 3042 }{ 39 } = 78 $$