So Paisley has 16 marbles total. You know that 8 of them are green, and thus the rest are yellow and orange. If you subtract the 8 green marbles from the total 16 marbles, that is 8 marbles left that can be yellow or orange. Assuming you mean that the yellow marbles have to make up 1/4 of all the marbles in the bag, you have to divide 16 by 4, getting 4. So, four of the marbles are yellow. If on the other hand, you mean the yellow marbles have to make up only what's left after subtracting the green marbles, you divide the amount of marbles left (8) by four, getting two marbles. I wasn't sure which you meant. Hope this helps!
