#### Answer

a. $0.5\%$
b. $26.2\%$

#### Work Step by Step

a.
To estimate the yearly increase, we need to:
1. Subtract the percentage from the first recorded year (1980) from the last recorded year (2010).
2. Divide the result by the number of years in between them.
$21.2-6.7=14.5$
$14.5\div30\approx0.5$
(We're rounding to the second digit, as instructed in the exercise).
The estimated average yearly increase is $0.5\%$.
b.
To estimate the percentage in 2020, we use the average yearly increase we just calculated and the last recorded year.
We need to multiply the difference in years by the average increase and add it to the percentage in 2010.
$2020-2010=10$
$21.2+10*0.5=26.2$
The estimated percentage in 2020 is $26.2\%$.