Check the picture below.
so the bag full of gold is 112Kgs, one quarter of that is 28Kgs, but hold the mayo!! our one quarter of gold only bag is not 28Kgs is really 28Kgs and 252grams extra, well, we know the gold reduced by one quarter, so that'd be the 28Kgs, and the slack is just the red part in the picture, the empty part atop of the bag, the 3/4 of the bag.
Now, we know 3/4 of the bag is 252grams, so if we divide the 252grams by 3, we'll know how much each of those three quarters weight.
Let me put it another way, we have four quarters, four of the same, we have three of the same above in red, we want to know how many grams one of those three of the same weight, well, since all three are 252g, one third of that must be what one weights.
