L = α * R where L is the length of the arc of the circle, R is the radius of the circle, and α is the central angle expressed in radians, so
[tex]R = \cfrac{L}{ \alpha }= \cfrac{10 \pi }{ \frac{5 \pi }{6} } = \cfrac{10 \pi *6}{5 \pi }=2*6=12 \ units [/tex]
