The answer would be 18 L
You do this by doing 48/8= 6
The reason you do 48 / 8 is because you you want to find out how many liters you need so if a 3L serves 8 people you would have to divid 48 into 8. (hope that makes since)
6 x 3 = 18L
Then take the 6 and times that by the 3 getting a total of 18 liters
(Hope this makes since :D)