Ratio word problems and proportions dealing with word problems.
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.