A trader bought 640 kg of rice. He sold a part of rice at 20% profit and the rest at 5% loss. He earned a profit of 15% in the entire transaction. What is the quantity (in kg) of rice that he sold at 5% loss?

128

Quantity of rice = 640 kg 

Overall profit percent = 15% 

                 20%                 -5%

                              15%

                 20%                 5%

                 4          :           1  = Total = 5

5x = 640 

1x = 128kg