in order to get your answer you have to get both fractions with common denominators and the common denominator would be 21 so then all you have to do is multiply 4/7 by 3 on both top and bottom of the fraction to get 12/21.
then you you take 12/21 - 4/21 and you keep the denominator to get 8/21.
so you should have 4 more blue marbles then red marbles.
