The strength of a bar magnet's magnetic field decreases with distance according to what is known as an inverse cube law; multiplying the distance from the magnet by a factor of k divides the magnetic field strength by a factor of k^3. Suppose the magnetic field strength of the magnet is 64 Gauss at a distance of 10 cm away. What is the magnetic field strength of the magnet, 20 cm away?
We multiply the distance by a factor of $2$, and thus divide the magnetic field strength by a factor of $2^3 = 2 \cdot 2 \cdot 2 = 8$. We get a magnetic field strength of 64/8 = 8
