Two coils having self-inductances of 0·09H and 0·1H and

Two coils having self-inductances of 0·09H and 0·1H and a mutual inductance of 0·015H. The coefficient of coupling between the coils is—

From relation, mutual inductance:
M = K L₁L₂
Given,
M = 0·015H, L₁ = 0·09H, L₂ = 0·01H
K=?
Now, 0·015 = K 0·09 × 0·01
or
K = 0·5