Let's say we want to reduce:
21
----
35
Here's an example:
Fraction: 21/35.
Factors:
21: {1, 3, 7, 21}.
35: {1, 5, 7, 35}.
GCF: 7.
Divide the numerator and the denominator:
21/7 = 3.
35/7 = 5.
Answer: 3/5.