In 2010 a algebra book cost $125. In 2015 the book cost $205. Whats the linear function since 2010?

In 2010 a algebra book cost $125. In 2015 the book cost $205. Whats the linear function since 2010?

In 5 years, the book appreciated 205 - 125 = 80 in value.

80/5 = 16.

So each year, the book increases 16 in value. Set up the cost function:
C(y) = 16y where y is the number of years since 2010