A fraction is considered to be "simplified" when it is expressed in the lowest term. That means the only common divisor between the numerator and denominator is 1, and no other.
Methods in Simplifying Fraction 
Method 1: By Repeated Division 
Method 2: By Greatest Common Factor 
* Keep dividing the numerator and denominator by a common divisor until such time that the only remaining common divisor is 1.
** Although there is no right way which common divisor to use in the beginning, I would suggest to use the first five (5) prime numbers in order as possible common divisor:

* Find the greatest common factor (GCF) of the numerator and denominator.
** Divide the top and bottom numbers of the fraction by the GCF to reduce to the lowest term.
*** You can find the GCF either by trial and error when the numbers are relatively small, or using Prime Factorization. 
This is a simple illustration showing the fraction is being reduced to its simplest form. Can you see a pattern?
Original fraction 
Not yet! 
Simplest form 



Let's go over a few more examples with detailed explanations.
Example 1: Simplify the fraction .
Simplify using Method 1: Repeated Division Method
It is obvious that 1 is not the only common divisor between the numerator and denominator. Since they are both even numbers, they must be divisible by 2.
 Divide the top and bottom by 2. Here's what we got after doing so.
The output fraction after dividing the top and bottom by 2 is . Can we stop here? Not yet! They can still be reduced by a second division of 2.
 Divide again the top and bottom by 2. The answer is (as the simplest form of ) because the only divisor of its numerator and denominator is 1. It's like the deadend of possible common divisor.
Simplify using Method 2: Greatest Common Factor Method
In the above solution using repeated division, we have simplified by dividing its numerator and denominator two times by the number 2. But wait! Is there a shortcut? Some of you may have observed that using a common divisor of 4 can directly simplify it with a single step!
 In fact, the Greatest Common Factor (GCF) of this fraction is 4 because it is the LARGEST number that evenly divides the numerator and denominator. Because the numbers are small, the GCF can be determined by trial and error.
Example 2: Simplify the fraction .
Simplify using Method 1: Repeated Division Method
Start simplifying using using the first few prime numbers (2, 3, 5, 7, 11, etc).
 Divide the top and bottom numbers by the first prime number which is 2.
 We still have a common divisor! Divide the top and bottom by the next larger prime number which is 3. We should get the final answer after this step.
Simplify using Method 2: Greatest Common Factor Method
To find the greatest common divisor, we are going to prime factorize each number. Next, identify the common factors between them. Finally, multiply the common factors to get the required GCF that can simplify the fraction.
Since GCF = 6, use this number to divide the numerator and denominator to get the answer in single step.
Example 3: Simplify the fraction .
Simplify using Method 1: Repeated Division Method
We can start testing numbers 2, 3 , 5, etc to simplify this. But there is an obvious divisor that stands out! Since both numbers end with zero, they should be divisible by 10.
Now, 2 can't divide both and so try 3.
Simplify using Method 2: Greatest Common Factor Method
Prime factorize each number and get the product of the common factors to obtain the needed GCF.
Simplify the given fraction in onestep using the divisor GCF = 30.
Example 4: Simplify the fraction .
Solution:
Divide the numerator and denominator by a common divisor of 3.
Example 5: Simplify the fraction .
Solution:
Simplify using repeated division method.
 Divide both numerator and denominator by 3, two times!
Example 6: Simplify the fraction .
Solution:
Simplify this fraction by greatest common factor method.
 Find the GCF by prime factoring both the numerator and denominator. Identify the common factors. Multiply them together to get the required GCF.
 After determining the GCF, divide the numerator and denominator to get the final answer.
Example 7: Simplify the fraction .
Solution:
Find the greatest common factor between the numerator and denominator, and use this number to simplify the fraction.
 Divide the numerator and denominator by GCF = 21.
