Determine the greatest common factor of the numerator (top number) and the denominator (bottom number). If you don't know the greatest common factor right away, choose the largest number that you know will divide into the numerator and denominator evenly. If you need to and are allowed, use a calculator to check this.
Divide the numerator and denominator by the greatest common factor.
Rewrite the fraction as the answer that you arrived at in Step 2. For example, if you have the fraction 40/50, you can divide 10 into both numbers evenly, leaving 4/5. This is the fraction's simplest form.
Rewrite the numerator, denominator or both as products of prime numbers if necessary. In this example, "5" is a prime number, but "4" is not. Since 4 is divisible by 2, rewrite the numerator as (2 * 2). Therefore, the fraction would be (2 * 2)/5. Another example of a fraction in simplest form that would be rewritten as prime numbers is 8/9. You would write this fraction as (2 * 2 * 2)/9.