Self-inductance of coil 1 (L1) = 15 mH Let the self-inductance of coil 2 is L2 Let the mutual inductance is M Total inductance in case of series addition = 70 mH ⇒15+L2+2M=70 ⇒L2+2M=55 Total inductance in case of series opposition = 30 mH ⇒15+L2−2M=30 ⇒L2−2M=15 From the above two equations, L2=35mH