A producer of accesories has production costs described by the function:

C= 800 - 10x + 1/4*x^2

x is the number of accesories produced per day.

Find the number of accesories needed to be produced on a daily basis in order to have the least cost.

a) 10

b) 20

Through substitution 20 seems to be the right answer, but is there other way to know?