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
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
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