If all the 4 kilograms that Teacher Zhang bought were mangoes, it would cost (6) yuan more. If you buy 4 kilograms of bananas, you will spend (6) yuan less.

2,30× 8 = 240 kg

20× 8 =160kg

If you buy 8 bags full of rice, there are (240) kilograms; If you buy 8 bags full of flour, you will get (160) kg.