1/3 of plain cookies + plain cookies = 4 parts. 60/4=15
there are 15 raisin, and 45 plain. Lets say chantalle ate x cookies, for all 0
That means there are 15-x raisin left, and 45-x plain left.
(45-x)-(15-x)=45-x-15+x=30. No matter the amount chantalle eats, the difference will always be $\boxed{30}$