Answer
$16$ millions per year in 1920.
$79$ millions per year in 2000.
Work Step by Step
Step 1. Calculate the slope of the secant line in the interval of $[1910, 1920]$ as $R_1=\frac{1860-1750}{1920-1910}=11$ millions per year.
Step 2. Calculate the slope of the secant line in the interval of $[1920, 1930]$ as $R_2=\frac{2070-1860}{1930-1920}=21$ millions per year.
Step 3. Estimate the rate of population growth in 1920 by averaging the slopes of two secant lines as $R_a=\frac{21+11}{2}=16$ millions per year.
Step 4. Calculate the slope of the secant line in the interval of $[1990, 2000]$ as $R_3=\frac{6090-5290}{2000-1990}=80$ millions per year.
Step 5. Calculate the slope of the secant line in the interval of $[2000, 2010]$ as $R_4=\frac{6870-6090}{2010-2000}=78$ millions per year.
Step 6. Estimate the rate of population growth in 2000 by averaging the slopes of two secant lines as $R_b=\frac{80+78}{2}=79$ millions per year.
