In order to sustain a steady catch each year, a limit of x(15x)x million fish are to be caught, leaving x million fish to reproduce each year. What is the number of fish which should be left to reproduce each year so that the maximum catch may be sustained from year to year?
1 Answer

let y be the number of catch each year
y=x(15x)x=15xx^2x=14xx^2
dy/dx=142x=0
x=7 million fish