#### Answer

a. $1.33$
c. $C=310+x*1.33$
b. $443$

#### Work Step by Step

a.
To estimate the yearly increase in the average carbon dioxide concentration, in parts per million, we need to:
1. subtract the number from the first recorded year (1950) from the last recorded year (2010).
2. divide the result by the number of years between them.
$390-310=80$
$80\div60\approx1.33$
(We're rounding the result to the nearest hundredth, as instructed in the exercise).
The estimated average yearly increase is $1.33$ (parts per million).
b.
The mathematical model looks like this:
$C=310+x*1.33$
C is the concentration of carbon dioxide.
310 is the value in 1950.
x is the number of years after 1950 for which we are calculating.
$1.33$ is the estimated average yearly increase.
c.
The average concentration in 2050, calculated with our mathematical model, would be:
$C=310+(2050-1950)*1.33=310+133=443$