Answer:
Finding the missing number is fairly simple and can be done in a few short steps. I will show you using the 21 / __ fraction.
First, divide the included number by its corresponding number in the original fraction.
21 ÷ 3 = 7
To find what the denominator of this would be, multiply 4 by 7.
4 * 7 = 28
The fraction would be 21 / 28.
This works with both numerators and denominators. If you need more of the answers just ask in the comments.
