The oil refinery makes low-sulfur and high-sulfar fuel. Every ton of low-sulfur fuel needs five minutes in blending plant and four minutes in refining plant; each ton of high-sulfur needs 4 minutes in blending plant and two minutes in refining plant. If blending plant is available for three hours and refining plant is available for two hours, how many tons of each kind of fuel must be manufactured so that plants are completely used?