The temperature (in degrees Celsius) as a function of time (in hours since noon) for a certain day in San Jose, Costa Rica, is graphed. What is the approximate average rate at which the temperature decreases between 3{:}30\text{ pm}3:30 pm3, colon, 30, start text, space, p, m, end text and 8{:}30\text{ pm}8:30 pm8, colon, 30, start text, space, p, m, end text?