The length $L$ of a metal rod is given in terms of the temperature $T\left(^{\circ} \mathrm{C}\right)$ by $L=39.6+0.025 T \mathrm{cm} .$ Within what range of temperature must the rod be kept if its length must be maintained within $\pm 1 \mathrm{mm}$ of $40 \mathrm{cm} ?$