Let's say we want to reduce:
12
----
15
Here's an example:
Fraction: 12/15.
Factors:
12: {1, 2, 3, 4, 6, 12}.
15: {1, 3, 5, 15}.
GCF: 3.
Divide the numerator and the denominator:
12/3 = 4.
15/3 = 5.
Answer: 4/5.