Here is the formula:
[tex]\frac{s}{angle} = \frac{r}{1}[/tex]
s = arc length
r = radius
angle = angle, but in radian form which is what the problem originally gave you so you don't have to do any conversions
so...
[tex]\frac{\frac{10\pi }{3} }{\frac{2\pi }{3} } = \frac{?}{1}[/tex]
Then cross multiply and you get...
[tex]\frac{10\pi }{3} =\frac{2\pi }{3} x[/tex]
Multiply [tex]\frac{3}{2\pi }[/tex] to both sides to isolate x and you will get...
5 = r
The radius is 5
Hope this helped!
