Convert : 18 seconds = \(\dfrac{18}{60}\) minute = \(\dfrac{3}{10}\) minute

The wheel takes 1 minute to travel : \(15:\dfrac{3}{10}=50\) (feet)

The perimeter of the wheel is : \(50:10=5\) (feet)

The diameter of the wheel is : \(\dfrac{5}{\pi}=1.5915494..\approx1.6\) (feet)

Change : 18 seconds = \(\dfrac{18}{60}\left(min\right)=0,3\left(min\right)\)

In 1 minute , the wheel travel : \(15:\dfrac{3}{10}=\dfrac{15.10}{3}=5.10=50\left(feet\right)\)

The perimeter of the wheel is: \(\dfrac{50}{10}=5\left(feet\right)\)

The diameter of the wheel is : \(\dfrac{5}{pi}=\dfrac{5}{3,14}=1,59....\approx1,6\left(feet\right)\)