At clock time t = 10 sec the illumination of a source is 68 watts/m^2, while at clock time t = 22 sec the illumination is 50 watts/m^2. Plot the corresponding points on a graph of illumination vs. clock time and determine the slope of the straight line segment connecting these points. Explain why this slope represents the rate at which the illumination changes over this time interval.
For the power function y = f(t) = 87 t-2, determine the average rate of change of y with respect to t, between clock times t = 22 and t = 27.