How many revolutions will the disk have made when it rolls a distance equal to the circumference of the disk (assuming no slippage)?
Let's check your answer using the arc-length formula:
[MATH]\theta=\frac{s}{r}=\frac{20\text{ ft}}{2\text{ ft}}=10\cdot\frac{1\text{ rev}}{2\pi}=\frac{5}{\pi}\text{rev}\quad\checkmark[/MATH]