In your marble collection 4/7 of the marbles are blue and 4/21 are red. how many times more blue marbles do you have than red ones?

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.