At the beginning of the week, a person bought a litre carton of milk for [[$ 4.20 $]] QAR, from which he had time to use [[$ 8 $]] dl before the milk was acidified. The following week, she bought two half-litre cartons of milk for [[$ 2.50 $]] QAR and used all of the milk. In which week did the milk she used cost less for her, and by what percentage? (Adapted from Finnish matriculation exam, spring 1995)

The price of a litre of milk used was in the first week

Answer: QAR

The price per litre in the second week was

Answer: QAR



Answer: The milk used during the second week was [[$\%$]] cheaper.