LCM( 15, 20 ) = 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 = 15, b = 20 and GCD(15, 20) = 5.
$$ \text{LCM} (15, 20) = \frac{ 15 \cdot 20 }{ \text{GCD}(15,20)} = \frac{ 300 }{ 5 } = 60 $$