Answer
$q=-2p+273$
Predicted sales at $\$ 75/$phone:$\qquad 123$ million phones
Work Step by Step
A demand function expresses demand $q$ (the number of items demanded)
as a function of the unit price $p$ (the price per item).
If it is a linear function, it has form $q=mp+b$.
Given $(85,103)$ and $(81,111)$, two points on the graph, we find the slope:
$m=\displaystyle \frac{111-103}{81-85}=\frac{8}{-4}=-2$
So far, we have $q=-2p+b.$
Substitute the coordinates of $(85,103)$ and solve for b.
$103=-2(85)+b$
$103=-170+b\qquad/+170$
$273=b$
Thus,
$q=-2p+273$
If $p=75,$ then
$q=-2(75)+273=123$ million phones.
