Time management is very much important in IIT JAM. The eduncle test series for IIT JAM Mathematical Statistics helped me a lot in this portion. I am very thankful to the test series I bought from eduncle.
Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Somnath
Don't get cofused with the word needle.You have to find the magnetic moment of the magnetized steel wire when it bent ti form a semicircle. When it bent to a semicircle the distance between the two end of the wire changes. The distance between the two end of the wire(two pole pieces) is now the diameter(2r) of the wire. For details calculation see the attached file.