(x-40)[500-(x-50)/ 10 * 100]= 8000
∴(x-40)( 1000- 10x)=8000
∴x？ - 140x+4800=0
That is, (x-60)(x-80)=0.
When x=60 or x=80, the monthly sales volume is 400 pieces, and the monthly sales profit reaches 8000 yuan.