Two coils of self-inductances 2 mH and 8 mH are placed so close together that the effective flux in one coil is completely linked with the other. The mutual inductance between these coils is

When the total flux associated with one coil links with the other, i.e., a case of maximum flux linkage, then M12=N2ϕB2i1 and  M21=N1ϕB1i2Similarly, L1=N1ϕB1i1 and L2=N2ϕB2i2 If all the flux of coil 2 links coil I and vice versa, then Since, M12 = M21 = M, hence we have M12M21=M2=N1N2ϕB1ϕB2i1i2=L1L2 Mmax=L1L2Given,  L1=2mH,L2=8mHMmax=2×8=16=4mH