An important concern in the study of heat transfer is to determine the steady-state temperature distribution of a thin plate when the temperature around the boundary is known. Assume the plate shown in the figure represents a cross section of a metal beam, with negligible heat flow in the direction perpendicular to the plate. Let $T_{1}, \ldots, T_{4}$ denote the temperatures at the four interior nodes of the mesh in the figure. The temperature at a node is approximately equal to the average of the four nearest nodes - to the left, above, to the right, and below.' For instance,
$$
T_{1}=\left(10+20+T_{2}+T_{4}\right) / 4, \quad \text { or } \quad 4 T_{1}-T_{2}-T_{4}=30
$$
(PICTURE NOT COPY)
Write a system of four equations whose solution gives estimates for the temperatures $T_{1}, \ldots, T_{4}$