How do you simplify the fraction #3/21#?
Find the greatest common factor of each term.
3 is a prime number, so in this case, 1 or 3 are the only valid candidates. 1 is not helpful though, so it is discarded.
Try dividing 21 by 3. If you get a whole number, it works. If you do not, that means you cannot simplify the fraction in the first place. If you get a whole number which is not a prime number and the numerator is not a prime number, it means you can simplify it further still.
In this case, 1 cannot be simplified further.
7 is a prime number as well.
Therefore, the answer is