notice, you've got a diameter in inches
and a linear velocity in miles
thus, you'd need to convert first
[tex]\bf v=rw\implies \cfrac{v}{r}=w\qquad
\begin{cases}
v=\textit{linear speed}\\
r=radius\\
w=\textit{angular speed}\\
--------------\\
v=20\frac{mi}{h}\\
r=\frac{diameter}{2}=\frac{27}{2}
\end{cases}
\\\\
\textit{now, converting 20miles to inches}
\\\\
\cfrac{20mi}{h}\cdot \cfrac{5280ft}{mi}\cdot \cfrac{12in}{ft}\implies \cfrac{20\cdot 5280\cdot 12\ in}{h}\impliedby v
\\\\\\
\cfrac{1267200\frac{in}{h}}{\frac{27}{2}in}=w[/tex]
now, whatever that is, you'll need to divide it by a revolution, recall, a revolution is [tex]2\pi[/tex]
thus [tex]\bf \cfrac{\qquad \boxed{\cfrac{1267200\frac{in}{h}}{\frac{27}{2}in}}\qquad }{2\pi }=revolutions[/tex]
