Hi,
Answer: 0.24 per liter
My work: This problem is easily achievable all you have to do is take your caculator and take price Melvin would pay with his loyalty card 1.679 and subtract it with 1.439. (1.679 - 1.439) When you do this you easily achieve your answer; Melvin pays 0.24 less per liter using his loyalty card.
I Hope it helped!