LCM( 15, 18 ) = 90
The formula for finding LCM is
$$ \text{LCM} (a, b) = \frac{ a \cdot b }{ \text{GCD}(a,b)} $$In this example we have: a = 15, b = 18 and GCD(15, 18) = 3.
$$ \text{LCM} (15, 18) = \frac{ 15 \cdot 18 }{ \text{GCD}(15,18)} = \frac{ 270 }{ 3 } = 90 $$