A small generator burns two types of fuel : low sulfur and high sulfur, to produce electricity. For one hour, each gallon of low sulfur emits 3 units of sulfur dioxide, generates 4 kilowatts of electricity and costs P60. Each gallon of high sulfur emits 5 units of sulfur dioxide, generates 4 kilowatts of electricity and costs P50. The environmental protection agency insists that the maximum of sulfur dioxide that can be emitted per hour is 15 units. At least 16 kilowatts must be generated per hour. The target is to get the least cost of fuel for the small generator.

What should be the definition of variable x ?