If the cooling system in a light-water nuclear reactor is shut off, the temperature of the fuel rods will increase. The temperature of the fuel rods during the first hour could be modeled by the equation $T=680(1.0004)^t−655$, where $t$ is the time in seconds, and $T$ is the temperature of the fuel rods in degrees Fahrenheit. Average rate of change can be calculated by finding the slope between two points. Find the average rate at which the temperature changes for the first $30$ minutes.