At two points separated by a distance of 3cm, the magnetic field strength due to a short magnet on its axial line are in the ratio 512:125. Then the distance of magnet from the first point is:
d1=d
d2=(d+3)
B1B1=512125
B=μ0m4πd3
=512125
(d2d1)3=512125
d2d1=85
d+3d=85
d=5 cm