On his farm, Old MacDonald can get thirty 30 cubic-metres of wheat per hectare of land if he does not use fertiliser. If he applies x kilograms of fertiliser per hectare of land, the marginal product of fertiliser is \left ( 1-\frac{x}{200} ight ) cubic-metres of wheat per kilogram of fertiliser. Suppose that Old MacDonald is a price taker in both the wheat market and the fertiliser market. The price of wheat is three dollars per cubic-metre. The price of fertiliser is thirty cents per kilogram. How many kilograms of fertiliser per hectare of land should be used by Old MacDonald in order to maximise his profits