Inductors in series (no mutual coupling): $L_{\text{eq}} = L_1 + L_2 + ...$. With mutual coupling: $L_{\text{eq}} = L_1 + L_2 \pm 2M$ (positive if fields aid, negative if they oppose). Inductors in parallel (no mutual coupling): $1/L_{\text{eq}} = 1/L_1 + 1/L_2 + ...$. These rules mirror resistors (series adds, parallel uses reciprocals). The sign of $2M$ in series depends on whether the coils are wound in the same or opposite directions — a common trap question.