Finding the Greatest Common Factor
Basics on the topic Finding the Greatest Common Factor
Learn how to find the greatest common factor using factor pairs and factor trees with this video.
Transcript Finding the Greatest Common Factor
Luis and June seem to be struggling to find some common factors between them. Speaking of common factors, let's learn about finding the greatest common factor. The Greatest Common Factor, or GCF, is the largest number that divides equally into two or more numbers. We often use the GCF to solve problems involving equal sharing, like dividing a cake or distributing supplies between your friends. There are two methods of finding the GCF; factor pairs and factor trees. First, let's explore factor pairs. This method lists out factor pairs and is most useful for smaller numbers. For this strategy, we will use twelve and fifteen. Factor pairs for twelve are one and twelve, two and six, and three and four. Factor pairs for fifteen are one and fifteen, and three and five. Now find the largest factor that both numbers have in common. What is the highest factor twelve and fifteen have in common? Three, so the GCF of twelve and fifteen is three. Now let's explore the second method, factor trees which uses prime factorization. This method is most useful when dealing with larger numbers. First, start with the numbers at the top; here we have twentyfour and thirtysix. Now find the prime factorization for each number. For twentyfour, we have two and twelve, two and six, and two and three. For thirtysix, we have two and eighteen, two and nine, and three and three. Next, identify the shared prime factors. Both twentyfour and thirtysix share a two, another two, and a three. Finally, multiply these together to find the GCF. We need to solve two times two times three. Two times two equals four, and four times three equals twelve. The GCF of twentyfour and thirtysix is twelve. Now it's your turn! Find the GCF of thirtysix and fiftyfour. Pause the video to work on the problem, and press play when you are ready for the solution. The factor tree of thirtysix is four and nine, two and two, and three and three. The factor tree of fiftyfour is six and nine, two and three, and three and three. The shared factors are a two, a three, and another three. Multiplied together you get a GCF of eighteen. Let's summarize. There are two useful methods for finding the greatest common factor, or GCF. The first method is factor pairs. This is useful for finding the GCF with smaller numbers. The second method is factor trees. This is useful for finding the GCF with larger numbers. Finding the GCF can help in making fair and efficient decisions when it comes to sharing. Ah, it looks like Luis and June have finally found their greatest common factor, their love for math and factor trees!
Finding the Greatest Common Factor exercise

What is GCF?
HintsWhen finding the GCF, we are looking for something that two or more numbers share or have in common.
The GCF of 12 and 15 is 3. This is the biggest number that fits equally into 12 and 15.
SolutionGCF stands for Greatest Common Factor.
It is the largest number that divides equally into two or more numbers.

Find the GCF of 12 and 24.
HintsAll of the factors of 12 are also factors of 24, which is the greatest?
Greatest means the largest common factor.
SolutionThe Greatest Common Factor (GCF) of 12 and 24 is 12.
This is the largest number that equally divides into both numbers.

Find the GCF of 54 and 72.
HintsFill in the gaps in each factor tree first.
How many times does 27 go into 54? This is a factor pair.
Once you have completed each factor tree, look for the shared factors.
You should find the shared factors of 2, 3 and 3. Multiply these together to find the GCF.
SolutionThe greatest common factor of 54 and 72 is 18.
When we complete the factor trees for 54 and 72, we can find the shared factors of 2, 3 and 3. When we multiply 2 $\times$ 3 $\times$ 3 we get 18.

Can you find the GCF of these pairs of numbers?
HintsDraw out factor pairs for smaller numbers or factor trees for larger numbers.
For 15 and 30, draw factor pairs. Here is one for 15, can you draw one for 30 and then find the GCF?
For 126 and 154, factor trees would be helpful.
Once you have drawn your factor trees, look for all of the shared factors and multiply them to find the GCF.
Solution The GCF of 15 and 30 is 15.
 The GCF of 84 and 144 is 12.
 The GCF of 126 and 154 is 14.
 The GCF of 21 and 35 is 7.

Find the GCF of 12 and 18.
HintsFirst look for common factors. Which numbers are factors of both 12 and 18?
Out of these common factors, which one is the greatest or largest number?
SolutionThe correct answer is 6.
The common factors are: 1, 2, 3 and 6.
Out of these common factors, 6 is the greatest.

Can you solve the problem?
HintsYou need to find the greatest common factor of 72, 96 and 104.
Draw a factor tree for each of these numbers and look for the shared factors.
You should find the shared factors of 2, 2 and 2. Multiply these together to find the answer.
SolutionThe greatest number of bunches is 8.
The greatest common factor of 72, 96 and 108 is 8, therefore, this is the greatest number of bunches Cara can make with the same number of of each colour of balloon.
Even and odd numbers
Divisibility Rules  3, 6, 9
Divisibility Rules  7
Divisibility Rules  4, 5, 8, 10
Prime Numbers
Integers and their Opposites
Least Common Multiples
Prime Factorization
Adding and Subtracting Rational Numbers on a Number Line
Ordering Rational Numbers
Cube Roots
Rational and Irrational Numbers
Ordered Pairs on the Coordinate Plane
Finding the Greatest Common Factor
Adding and Subtracting Decimals
Comparing Fractions
Equivalent Fractions
Simplifying Fractions
Temperature Conversion
Division with Exponents
Multiplication with Exponents
Improper Fractions and Mixed Numbers
Multiplying Mixed Numbers: Word Problems