Question 458643
Suppose you have a cookie stand, and when you charge $3 per cookie
box you sell 200 boxes. But when you raise your price to $4 you only
sell 120 boxes. 
a. Write an equation for the number of boxes you sell as a function of
the price you charge. 
b. Denote "B" for number of boxes, and "P" for the price you charge. 
c. Assume the function is linear.
..
Since function is linear, we will be using the standard form: y=mx+b, with m=slope, b=y-intercept
For given problem:
B=mP+b
Given are two points on the (P,B) curve,(3,200) and (4,120)
slope, m=∆B/∆P=(200-120)/(3-4)=80/-1=-80
B=-80P+b
plug in B & P from one of given points to solve for b, the y-intercept
200=-80*3+b
b=200+240=440
Equation: B=-80P+440