This question uses the concept of lowest common multiple which I believe is taught only in secondary school. But for now, just understand that in order to find the smallest possible difference, you need to first find the minimum number of apples that has to be bought so that there’s an equal number of red and green apples. In this case, if u buy 4 bags of 3 red apples and 3 bags of 4 green apples you get 12 apples each. Hope this helps :)