The temperature distribution in a fluid is given by $T=10 x+5 y,$ where $x$ and $y$ are the horizontal and vertical coordinates in meters and $T$ is in degrees centigrade. Determine the time rate of change of temperature of a fluid particle traveling (a) horizontally with $u=20 \mathrm{m} / \mathrm{s}, v=0$ or $(\mathrm{b})$ vertically with $u=0, v=20 \mathrm{m} / \mathrm{s}$.