EXPLANATION
If the circunference of the wheel is of 63 inches, then the diameter is given by the following relationship:
[tex]\text{Circunference}=\pi\cdot\text{diameter}[/tex]Isolating diameter:
[tex]\frac{Circunference}{\pi}=\text{Diameter}[/tex]Replacing terms:
[tex]\frac{63}{3.14}\approx20\text{ inches}[/tex]