we have
x ----> number of chapters
y ---> number of pages
In this problem
we have a proportional relationship between the variables x and y (because the line passes through the origin)
so
The linear equation is of the form
y=Kx
where k is the constant of proportionality
K=y/x
looking at the graph
we take the point (4,40)
so
k=40/4=10
the equation is
y=10x
For y=180
substitute
180=10x
solve for x
x=180/10
x=18
the answer is 18 chapters
