You can get 15 chocolates for 15 Rupees. With the wrappers you can get 15/3= 5 more chocolates. If you are allowed to use the wrappers from the “free” chocolate, you can get one more chocolate with two wrappers left over. That's 21 so far, but when you eat that last chocolate you now have 3 more wrappers. One more chocolate 21+1= 22 chocolates. One more chocolate gives a total of 22. And a stomachache.
