Magnetic field of a toroid

dangish

The toroid shown in the figure has a wire carrying a current I= 7.10 Amperes wrapped around it N= 820 times. The inner radius is R1 23.0 cm and outer radius R2 27.6cm.

Figure:
http://capaserv.physics.mun.ca/msuph...ob01_torus.gif


What is the magnitude of the magnetic field along a circle that is halfway between the inner and outer edges of the toroid?

Here is what I tried,

B = u0*I*r*N / (2Pi*R^2)

maybe I have to divide N by the current and replace the N in my equation above by N/I??

Please help

Redbelly98

That formula doesn't look right. Where did you get it from, and what are r and R?