A very long straight current-carrying wire produces a magnetic field of 20 mT at a distance d from the wire. To measure a field of 5 mT due to this wire, you would have to go to a distance from the wire of A- 4d.
B - 16d.
C - d sqrt2
D- 8d.
E- 2d.

A. 4d

Explanation:

Magnetic field strength is inversely proportional to distance. So in order to have a smaller magnetic field, we need to move further out from the wire. How far we go exactly can be determined from the formula: B=(μ₀I)/(2πr)

(That is derived from Ampere's Law, which states ∫B•dl=μ₀I)

With that you can set up a ratio between the magnetic fields in both cases. Because the current is the same for both instances, everything reduces out on one side of the equation and leaves you with something that relates the two distances by a ratio of each magnetic field value.

My work is in the attachment, comment for questions.

ill do it for 100, also this isnt 80 its 40

f is prop to m1xm2 double m1 double force