chandan1988, see if it helps:

In terms of the weight of bags, we could write the following equation: 65 <= 5*a + 10*b + 25*c <= 80 or 13 <= a + 2*b + 5*c <= 80 (equation I)

In terms of the total cost (T) we could write: 13.85*a + 20.43*b + 32.33*c = T, for which we want to minimize the value of T.

The main problem is that we have (a, 2b, 5c) in the first equation, and (a, b, c) in the second. But, what if we just transform the variables in the second equation to (a, 2b, 5c)?

T = 13.85*a + (20.43/2)*(2b) + (32.25/5)*(5c) = 13.82*a + 10.215*(2b) + 6.45*(5c) (equation II)

Thus, now we have a system of two equations, for which is clear that best thing to do is maximize c.

In other words, to solve it more quickly, we must think what kind of bag is the cheapest in terms of $ per pound.