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.