=(20+4x)(60-40-x)

=(20+4x)(20-x)

=400-20x+80x-4x^2

= -4x^2+60x+400

= -4x^2+60x-225+625

= -4(x^2- 15x+225/4) + 625

= -4(x- 15/2)^2 +625

Because (x- 15/2) 2 ≥ 0, -4 (x- 15/2) 2 ≤ 0.

So when x= 15/2=7.5 (yuan), (x- 15/2) 2 = 0, w max =625 yuan.

Note: 2/ 15 in the answer should be 15/2.