Current I is flowing in a coil and two points B1 and B2 are on the axis of coil at a distance 0.05 m and 0.2 meter respectively. If ratio of magnetic field at point B1 and B2 are in 8 : 1, then find radius of coil.
(1) 0.1 m
(2) 0.2 m
(3) 0.7 m
(4) 0.8 m