First method is just breaking it down for easier understanding. If 5% are rotten, and 20 apples rotten, means 5% equals to 20 apples. So to find 100%, can divide 20 by 5 to find out how many apples 1% represents. Then can find total number of apples from there. Second method is a shortcut, where we put the percentage we want to find (which is 100% in this case) over the current percentage (5%) multiplied by the value that current percentage represents (which is 20 apples)