At a store, carpet is sold in units of 10 square yards.
based on the graph, how much would 80 square yards of carpet cost?
a) $1350
b) $1200
c) $1050
d) $900

We can generalise this by finding the equation of the line.
The gradient is: 15

We can take when x = 10, and y = 150 to find the equation of the line.
y - 150 = 15(x - 10)
y - 150 = 15x - 150
y = 15x

Thus, when x = 80, y = $1200 (ie B)