For each of the following populations, the instantaneous rate of change of the population size at t

Question:

For each of the following populations, the instantaneous rate of change of the population size at t = 0 is exactly 1.0 million bacteria per hour. If you computed the average rate of change between t = 0 and t = Δt, how small would Δt have to be before your value was within 1 % of the instantaneous rate of change?
b(t) = t + 0.1t2.