The ratio of magnetic inductions on the axial line of a bar magnet at distances 10cm and 15cm from the center of the magnet are in the ratio 5.8: 1. The distance between the two poles of the magnet is

For long bar magnet, Baxial = μo4π2Md(d2−l2)2 ⇒B ∝ d(d2−l2)2⇒ 5.81 = 1015 152−l2102−l22 ⇒8.7 (102−l2)2= (152−l2)2 ⇒2.95 (100-l2) = (225-l2)⇒l2 = 295-2252.95-1=701.95≃36 ⇒ l=6 ⇒Distance between two poles= 2l = 12cm