Assuming that there is only 1 bag, there is a set value (we can call that b) for that cost. In addition, there is a set value for what the bag weighs. We can call that m. Therefore, for x pounds, we multiply the amount per pound by the amount of pounds (x) to get m*x+b=cost (y). In a line mx+b=y, m is the rate of change. m can be found by
[tex] \frac{y1-y2}{x1-x2} [/tex]
Using that, with the cost being the y values and the amount of pounds being the x values, we have (20, 25) at one point and (50, 40) at another. For our equation, we have
[tex] \frac{40-25}{50-20} = \frac{15}{30} [/tex]
Dividing both the numerator and denominator by 15 (we know they are both factors of 15 with a little guess and check), we have 1/2 or 0.5 as our answer
