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
