The expression for p(t) can be easier to evaluate if it is factored to
p(t) = t·(t+5)
a. The average rate of change (r) on the interval [0, 5] is
r = (p(5) - p(0))/(5 - 0)
r = (5·(5+5) - 0·(0+5))/5
r = 10 . . . . percent per day
b. Differentiating the equation, you have
p'(t) = 2t + 5
Evaluating that at t=2 gives
p'(2) = 2·2 + 5 = 9 . . . . . percent per day, instantaneous rate at t=2
Interpretation: on day 2, the rate of infection is 9% of the population per day.