LCM( 5, 20 ) = 20
The formula for finding LCM is
$$ \text{LCM} (a, b) = \frac{ a \cdot b }{ \text{GCD}(a,b)} $$In this example we have: a = 5, b = 20 and GCD(5, 20) = 5.
$$ \text{LCM} (5, 20) = \frac{ 5 \cdot 20 }{ \text{GCD}(5,20)} = \frac{ 100 }{ 5 } = 20 $$