The so-called gross profit refers to the balance of sales revenue MINUS the purchase cost, because it includes transportation costs, personnel wages and so on. So it's called gross profit, not net profit.

Solution: {5+[400-(8-5) * (200/2)]/(200/2)}/8 = 7.5/10.

So we should give you a 7.5 discount.

Description: After selling half of it, it has earned (8-5)*(200/2)=300 yuan.

If the gross profit wants 400 yuan, the remaining half needs to earn 400-(8-5)*(200/2)= 100 yuan.

The remaining100kg needs to earn [400-(8-5) * (200/2)]/(200/2) =1yuan per kg.

The remaining selling price should be 5+ 1=6 yuan.

From 8 yuan to 6 yuan, the discount rate is 6/8=0.75, which means you should get a 7.5 discount.