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?

Answer: 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.