Answer:
Let x be the number of kilograms of millet and y be the number of kilograms of grain.
We want to solve the system of equations:
x + y = 500 (total weight of birdseed)
0.28x + 0.18y = 0.24(500) (total cost of birdseed)
Simplifying the second equation:
0.28x + 0.18y = 120
Multiplying the first equation by 0.28 and subtracting it from the second equation, we get:
0.28x + 0.18y - 0.28x - 0.28y = 120 - 0.28(500)
-0.10y = -40
y = 400
Substituting y = 400 into the first equation, we get:
x + 400 = 500
x = 100
Therefore, the manufacturer should use 100 kg of millet and 400 kg of grain to make 500 kg of birdseed worth 24 cents a kilogram.