Magnetic Field Physics Help - Missing Equasion?

I have this question on a practice exam for intro physics (watered down to the basic information): At a distance 1.0km from a tornado, there is a constant magnetic field 1.35x10^-7T associated with it. What is the average current going down the funnel, if mu naught (I don't know how to type that, it's the... show more I have this question on a practice exam for intro physics (watered down to the basic information):
At a distance 1.0km from a tornado, there is a constant magnetic field 1.35x10^-7T associated with it. What is the average current going down the funnel, if mu naught (I don't know how to type that, it's the u-looking symbol) =4*pi*10^-7 Tm/A. The answer given is 675A, but I can't figure out how to get there. I tried to use B=mu naught*I/2*pi*r and solve for I, but I got 31.4A doing that. Any help would be appreciated!
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