How far from a long conductor passing a current of 12.5A is the magnetic field of strength 3.1x10^-5 T?
Presumably the conductor is a wire of some kind. (Rather than, say, a rectangular solid, which would distort the magnetic field lines.) What does Ampere's Law have to say about the magnetic field due to a long wire?
$\displaystyle B = \frac{\mu _ 0I}{2 \pi r}$
where r is the distance from the center of the wire.
-Dan
Ummm...
$\displaystyle \mu _0 = 4\pi \times 10^{-7} \frac{N}{A^2}$
The "permeability of free space?" If you are doing this problem you have to have run into this constant before!
Edit: Perhaps you know of your magnetic field as
$\displaystyle B = \frac{c^2I}{2\pi \epsilon _0 r}$
In that case we have the identity $\displaystyle c^2 = \epsilon _0 \mu _0$ where c is the speed of light.
-Dan