Well, you start out by recognizing that 4/5ths of an ounce equals 1/5 of the apples weight. Therefore, you must multiply 4/5 by 5 to get the whole of the Apple. (1/5*5).
So, this is the lines,
4/5*5= 20/5, or 4 ounce
Another way, is with decimals, so
0.8*5=4.0