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