We know that an inch is equivalent to 2.54 centimeters, then if we want to know how many inches are in a centimeters we do this:
[tex]a\times\frac{1inch}{2.54\operatorname{cm}}[/tex]In this case, we have 20 centimeters, then replacing a by 20 we find the equivalent inches to 20 like this:
[tex]20\text{cm}\times\frac{1inch}{2.54\operatorname{cm}}\approx7.87\text{inches}[/tex]