In order to play table games at the casino you need some tokens. Red tokens cost 7 euros and yellow tokens cost 4. Write a function buy_tokens(n) that given a number n of euros such that n ≥ 20, it returns the equivalence in tokens. When several equivalences are possible the function returns the one minimizing the total number of tokens.
>>> buy_tokens(20) (0, 5) >>> buy_tokens(50) (6, 2) >>> buy_tokens(39) (5, 1)