A thin rectangular plate, $0 \leq x \leq a, 0 \leq y \leq b$, with negligible heat loss from its sides, has a linear temperature variation along the edge at $y=b$ given by $T=20+100(x / a)^{\circ} \mathrm{C}$. The other three edges are maintained at $20^{\circ} \mathrm{C}$. Determine the temperature distribution $T(x, y)$.