Joe's annual income has been increasing each year by the same dollar amount. the first year his income was $20,400 and the 11th year his income was $31,400. in which year was his income $40,200?

18th year
In this question, the income is increasing each year with the same amount, first-year income is $20,400 and 11th-year income is $31,400. From this information, you can determine how much the income increases every year. The calculation would be:

Income year Y= first income + {(Y-1) * income increase per year )}
$31,400= $20,400 + 10 * income increase per year
10* income increase per year= $11000
income increase per year= $1100

Using the same formula you can find the answer

Income year Y= first income + {(Y-1) * income increase per year )}
$40,200= $20,400 + Y* 1100
1100Y= 40,200-20,400= 19800
Y= 19800/1100= 18 year.