Monthly sales profit =(55-40)*450=6750 yuan.
2.y=(x-40)[500- 10(x-50)]
3. Monthly sales volume
500- 10(x-50)& lt； = 250 x & gt=75
y =(x-40)[500- 10(x-50)]& gt； =8000
y = x^2- 140x+4800<； =0
=(x-60)(x-80)& lt； =0
At the age of 60