I believe the answer is 6 white marbles. how i figured that out was i made an equation. i took the variable Y (for the yellow marbles) times 2 minus 6 equals 18. you then add 6 to the other side of the equal sign to cancel out the 6. you then have y(2)=24. you divide y and 24 by 2. you get 12 yellow marbles. next step is to take 18 - 12. you are left with 6 white marbles. hope this helps!
