A long straight metal rod of radius 4 cm carries 2A, uniformly distributed over its circular cross section. Find the magnetic field induction at a distance 3 cm from the axis of the wire.

current through the 4cm cross section is 2A so the current through the 3cm cross section will be (3/4)*2 ampere now make a circular loop and using the ampere circutal law

B*2*pie*0.03=mu not times  (3/4)*2

instead of B all the quantities are known hence B can be easily found.