Another way to approach this question would be to consider how many equal parts are being distrubed. Looking at the ratio of 3:5 as "for every 3 marbles that Bill gets, Carl get 5," it is easier understand that the marbles will be split into 8 equal parts.
The total marbles: 48 ;
divided by the number of equal parts: 8
gives you 8 groups of 6 marble.
If Bill gets 3 of the groups (each having 6 marbles)
he has a total of 6*3 marbles or 18 marbles.