Solution: When the sales unit price is set to X yuan per kilogram, the monthly sales volume is [500-(x–50 )×10] kg, and the sales profit per kilogram is (x–40) yuan, then the monthly sales profit is:

y =(x–40)[500-(x–50)× 10]=(x–40)( 1000– 10x)=– 10x 2+ 1400 x-

The resolution function of y and x is: y =–10x2+1400x–40000.

Therefore, the monthly sales profit should reach 8000 yuan, that is, y=8000, ∴–10x2+1400x–40000 = 8000.

Namely: x2–140x+4800 = 0,

Solution: x 1=60, X2 = 80.

When the sales unit price is set at one kilogram of 80 yuan, the monthly sales volume is: 500-(80-50)× 10 = 200 (kg), and the monthly sales unit price cost is:

40×200=8000 (yuan);

Since 8000 < 10000 < 16000, and the monthly sales cost cannot exceed 10000 yuan, the sales unit price should be set at 80 yuan per kilogram.