The table gives the population of the world in the 20th century.
$$\begin{array}{|c|c|c|c|} \hline \mbox{Year} & \mbox{Population (in millions)} & \mbox{Year} & \mbox{Population (in millions)}\\ \hline 1900 & 1650 & 1960 & 3040 \\ 1910 & 1750 & 1970 & 3710 \\ 1920 & 1860 & 1980 & 4450 \\ 1930 & 2070 & 1990 & 5280 \\ 1940 & 2300 & 2000 & 6080 \\ 1950 & 2560 & & \\ \hline \end{array}$$

(a) Estimate the rate of population growth in 1920 by averaging the slopes of two secant lines (using the years 1910 and 1920, then 1920 and 1930).
Answer: million/year

(b) Repeat the same process for 1980.
Answer: million/year