Timothy bought 10 cookies for $5.00. let x represent the number of cookies purchased, and let y represent the total cost. graph the line that represents the proportional relationship.

Think:  Timothy could buy 0 cookies for $0.00.  Thus, (0,0) and (10,5) are two points on the "total cost" line.

This line passes thru the origin, which is necessary if this is a proportional relationship.
                                               $5-$0
The slope of this line is m = = $0.50/cookie
                                                10-0

and the total cost equation is then     C(x) = ($0.50/cookie)x