A sack contains red and blue marbles The ratio of red marbles to blue marbles is 43 If there are 16 red marbles in the sack how many blue marbles are in the sack?

This problem doesn't lead to a whole number solution. Are you sure you copied it correctly? Maybe you meant the ratio is 4 to 3. In that case:

red/blue = 4/3

blue/red = 3/4

blue = (3 * red) / 4 = (3 * 16) / 4 = 12 blue marbles