(1) A: This fruit supermarket buys the most apples, accounting for 40%; Oranges are the least, accounting for 10%.

(2)(40%-30%)÷30%

= 10%÷30%

≈33.3%

Answer: I bought about 33.3% more apples than bananas.

(3) How many kilograms of apples do you buy?

800×40%=320 (kg)

Answer: I bought 320 Jin of apples.