Profit y
y =(20-x- 10)( 100+20x)
= 20( 10 x)(5+x)
Because 10-x+5+x = 15- fixed value.
therefore
The product has a maximum value. When 10-x = 5+x,
that is
The maximum value is taken when x=2.5.
At this time, the maximum profit =20×7.5×7.5= 1 125 yuan.