-straight wire with radius R, carrying current I

-current is uniformly distributed across the cross sectional area of wire

-calculate the magnetic field inside wire as function of distance r from the center of the wire

In the solution, there is a picture of the cross section of the wire, and the current is going into the page, ie X.

there is an imaginary circle, "amperian loop" with radius r, inside the wire.

then, B*2*pi*r = u*i_inside (equation 1)

then somehow i_inside = I*(pi*r^2)/(pi*R^2) = I*r^2/R^2 (equation 2)

This i dont understand. I know it something to do with the fact that current is uniformly distibuted, but how to get equation 2?

Is it just a ratio?

Thanks!