Locomotive Coal Consumption and Journey Cost

The consumption of coal by a locomotive varies as the square of the velocity; when the speed is 16 miles an hour the consumption of coal per hour is 2 tons : if the price of coal be 10s. per ton, and the other expenses of the engine be 11s. 3d. an hour, find the least cost of a journey of 100 miles.

H. S. Hall & S. R. Knight, Higher Algebra (1891) · Ch. III. Variation, Examples III, No. 22