Video: Finding the Greatest Common Factor

Kathryn Kingham

We use prime factor decomposition to help us find the greatest common factor of two or three numbers. Multiplying together the prime factors that are common to all the numbers enables you to quickly calculate the value of the greatest common factor.


Video Transcript

Let’s take a look at finding the greatest common factor. But before we find the greatest common factor, we probably should talk about what the greatest common factor is. To define greatest common factor, let’s start here with the word factor. Factors are numbers that are multiplied together to produce another number. For example, with the number twelve, two times six equals twelve. Two and six are both factors of twelve. Three and four are also factors of twelve. It’s also possible that you’ve heard of something called a factor tree and that looks something like this. We use a factor tree when we are interested in finding prime factors. We’ll also use it today when we’re looking at finding greatest common factor. Okay, so now that we’ve talked about factor, we can talk about the greatest and common, and then put them all together. The word greatest here means just what you think it means: the largest, the biggest, the highest in numerical value. And the word common here means shared by all the numbers that you are comparing. So when we find the greatest common factor, we’ll be comparing two or more numbers. And we want something that is shared by all of them. When we put the definition together, we could say the greatest common factor is the largest shared number that can be divided into each number you are comparing. We should also note that many times we see the abbreviated form GCF, and that means greatest common factor.

Here’s an example. Find the greatest common factor of twelve and thirty. I’m gonna start by making a factor tree of the number twelve and the number thirty. Twelve is divisible by two and six. Two times three equals six, And now I have all the prime factors of twelve. Moving on to thirty, three times ten equals thirty. Two times five equals ten. And those are the prime factors. Once we find the factors of twelve and thirty, we can start to see common factors emerge. For example, three and three are both factors of twelve and thirty. The number two is a factor of twelve and a factor of thirty. But for us, the key is that we’re looking for the greatest common factor. We can find the greatest common factor by taking all the shared factors and multiplying them together. Since each number is divisible by two and three, we multiply two times three together to get six. Six is then in our greatest common factor. So the greatest common factor of twelve and thirty is six.

Our next example asks for the greatest common factor of nine and fourteen. Following the same procedure, let’s set up factor trees for nine and fourteen. Nine is divisible by three. Its prime factors are three and three. We’re finished there. And for fourteen, we have two and seven. Both of those are prime numbers, so that’s all we can do for our factor tree. Take a look at the factors of nine and fourteen. So something is happening here that is different from our first example. I know what you’re thinking, no common factors? So how does this question work if there are no common factors? But there actually is one number that is still a factor for both nine and fourteen. You can multiply nine and fourteen by one. You can also divide nine and fourteen by one, so the greatest common factor here is the number one. If numbers don’t share any other common factors, we know that they always share one.

To encourage public transportation, Leo wants to gift some of his friends envelopes with bus tickets and subway tickets in them. If he has thirty-two bus tickets and eighty subway tickets to split out equally among the envelopes and he wants to make sure no tickets are left over, what is the greatest number of envelopes Leo can make? For this problem, we’re gonna need to use, you guessed it, the greatest common factor. The greatest common factor of thirty-two and eighty will help us determine exactly how many envelopes Leo can make, because it will tell us how many groups we can divide each of these things into equally. We’ll start this process the same way we always do, finding the factors of thirty-two and eighty. Thirty-two is divisible by two and sixteen. Two times eight equals sixteen. Two times four equals eight. Two times two equals four. With eighty, I’m gonna start with eight and ten. Eight times ten equals eighty. Two times four equals eight. Two times two equals four. Two times five equals ten. Here are all of the factors, and now let’s figure out which ones are the common factors. Both thirty-two and eighty have four twos as they’re factors. Two to the fourth power is the greatest common factor of thirty-two and eighty, so we need to find out what is two to the fourth power. Two to the fourth power equals sixteen. This tells us that Leo can make sixteen envelopes. Leo can then give out sixteen envelopes. And in those sixteen envelopes, each envelope will have two bus passes and five subway tickets. They will all be equal. There will be seven things in each envelope: Two bus tickets, five subway tickets.

Let’s look at one other type of example. This example has three numbers that we are comparing and trying to find the greatest common factor of. We are gonna use the same process. So no matter if there’s three or four or five or six numbers, you’re trying to find the greatest common factor of. You’re going to use the same procedure that we’ve been using. We’re gonna use factor trees to find the greatest common factor. And so here we go with our factor trees. Ten times eleven is one hundred and ten. Two times five equals ten. And now for forty, four times ten, four equals two times two. Ten equals two times five; Twelve times ten equals one hundred and twenty. Two times six equals twelve. Two times five equals ten. Finally, two times three equals six. Each of these numbers have two and five in common, which means their greatest common factor is ten. We can also see this in this part of each factor tree. Two times five equals ten. Ten is the greatest common factor of these three numbers. And now it’s your turn to go out and try and find the greatest common factor of some numbers.

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