LCM( 20, 15 ) = 60
The formula for finding LCM is
$$ \text{LCM} (a, b) = \frac{ a \cdot b }{ \text{GCD}(a,b)} $$In this example we have: a = 20, b = 15 and GCD(20, 15) = 5.
$$ \text{LCM} (20, 15) = \frac{ 20 \cdot 15 }{ \text{GCD}(20,15)} = \frac{ 300 }{ 5 } = 60 $$