The room temperature is $+ 20^{\circ}C$ when outside temperature is $- 20^{\circ}C$. Room temperature is $+ 10^{\circ}C$ when outside temperature is $- 40^{\circ}C$. The temperature of the radiator heating the room is

Question

$\begin {array} {1 1} (1)\;70^{\circ}C & \quad (2)\;60^{\circ}C \\ (3)\;50^{\circ}C & \quad (4)\;40^{\circ}C \end {array}$