Abox half full of apples weighs 40kg and the same box one fifth full weighs 22kg. what does the box weigh when empty?

So in order to answer your question  we calculate 40 - 22 wich gives us the answer of 18, so now we can say that 4/5 of apples weight 18 so now 18 / 4 = 4,5; 4,5 * 5 = 22,5 wich is the weight of apples you can put in the box, to know the answer we deduct the greatest weight of apples only from the weight of the box with those apples so, 40 - 22,5 = 17,5
so the answer to your question is 17,5
i hope i helped you