posted by hassan .
A length of wire carrying a current i and bent into a circular coil of one turn.The same length of wire has been out to give a coil of 5 turns, each of 1\5 the original radius.If Ba and Bb are the magnitudes of the magnetic fields at the centers of the two coils, what is the ratio Bb\Ba?
The formula you need can be found here:
Multiply it by N if there are N loops.
Since the field at the center is inversely proportional to R, The field increases by N^2 . The ratio is 25.