slope formula
points (1,40) and (4,85)
m=[tex] \frac{y_{2}- y_{1} }{x_{2}-x_{1}} [/tex]
m=[tex] \frac{85-40}{4-1} [/tex]
m=[tex] \frac{45}{3}[/tex]
y=mx+b
y=40; x=1; m= 45/3
40=(45/3) + b
40 - (45/3)= b
b= 25
therefore
y=(45/3)x + 25
when x=0
y=25
so the value of the book was $25
