Because of cholesterol problems, I must take a certain medicine. Every week I buy a box, which lasts me for seven days.
However, things are not so bad. The laboratory that produces this medicine has an standing offer: for every five empty boxes, they give you a free new box.
Thus, I get six boxes paying just for five; it means a 16.67% savings for me... right or wrong?
The savings would be 20%, IF he had had a free empty box to start. As he didn't, his savings would be 20% only as a limit, if he lives forever.