Solution process

Let the monthly sales volume y and price x of this commodity satisfy a linear functional relationship y=kx+b,

Use points (20,360) and (25,210).

20k+b=360

25k+b=2 10

The solution is k =-30 and b = 960.

So y=-30x+960.

The monthly profit earned by the store is

p = x×(-30x+960)- 16×(-30x+960)

p=-30x？ +960x+480x- 15360

p=-30x？ + 1440x- 15360