Factorization and least common multiple questions appear frequently on the GRE exam. To factorize any number means to find the prime numbers that are factors of the original number. Factorization is simple if the original number is small. For example, 21 is the product of the prime numbers 3 and 7.
If you want to factor a large number, choose 2 factors and check to see if they are prime numbers. If neither or only one of them is a prime, factor the nonprime number(s). Continue this factorization process until you are left with only prime numbers.
As an example, factorization of the number 240 is done as follows:
Step 0: 
Original number 
240 
Step 1: 
Divide 240 by 10 
240 = 24 x 10 
Step 2: 
Divide 24 by 6 
240 = 4 x 6 x 10 
Step 3: 
Divide 4 by 2 
240 = 2 x 2 x 6 x 10 
Step 4: 
Divide 6 by 3 
240 = 2 x 2 x 3 x 2 x 10 
Step 5: 
Divide the 10 by 5 
240 = 2 x 2 x 3 x 2 x 5 x 2 
From the above steps, you see that 240 is factorized into 240 = 2^{4} x 3 x 5, and so the prime factors of 240 are 2, 3 and 5. Basically, notice that you keep on dividing numbers as long as there is a number left to divide. Once you are left with only prime numbers, then you know that you've completed the factorization.
The Least Common Multiple (LCM) of two or more integers is the smallest integer that is a multiple of each of them. For example, the LCM of 5 and 3 is 15, and the LCM of 22 and 3 is 66.
The Greatest Common Factor (GCF) of two or more integers is the greatest integer that is a factor of each of the original numbers. For example, the GCF of 12 and 9 is 3, because the prime factors of 12 are 3 and 2, and the factor of 9 is 3, and so 3 is the largest common factor.
Use the following sample question to practice:
Example GRE Question 
Which of the following are factors of 2013^{2}
 1
 11
 31
 61
 81
 91
 121
 131
 671
Solution: First express 2013 as a product of its prime factors:
2013 = 3 × 671 = 3 × 11 × 61
Therefore:
2013^{2} = 3^{2} × 11^{2} × 61^{2}
Using that information, the question now is relatively straightforward:
 1 is a factor of every whole number, thus Choice A is correct
 11 is a factor of 2013^{2}, thus Choice B is correct
 31 is prime and is not a factor of 2013^{2}, thus Choice C is incorrect
 61 is a factor of 2013^{2}, thus Choice D is correct
 81 = 3^{4} is not a factor of 2013^{2}, thus Choice E is incorrect
 91= 7 × 13 is not a factor of 2013^{2}, thus Choice F is incorrect
 121 = 11^{2} is a factor of 2013^{2}, thus Choice G is correct
 131 is prime and is not a factor of 2013^{2}, thus Choice H is incorrect
 671 = 11 × 61 is a factor of 2013^{2}, thus Choice I is correct
