# What is the Greatest Common Factor (GCF) of 1, 1, 21, and 15?

Are you on the hunt for the GCF of 1, 1, 21, and 15? Since you're on this page I'd guess so! In this quick guide, we'll walk you through how to calculate the greatest common factor for any numbers you need to check. Let's jump in!

Want to quickly learn or show students how to find the GCF of two or more numbers? Play this very quick and fun video now!

First off, if you're in a rush, here's the answer to the question **"what is the GCF of 1, 1, 21, and 15?"**:

GCF of 1, 1, 21, and 15 = 1

## What is the Greatest Common Factor?

Put simply, the GCF of a set of whole numbers is the largest positive integer (i.e whole number and not a decimal) that divides evenly into all of the numbers in the set. It's also commonly known as:

- Greatest Common Denominator (GCD)
- Highest Common Factor (HCF)
- Greatest Common Divisor (GCD)

There are a number of different ways to calculate the GCF of a set of numbers depending how many numbers you have and how large they are.

For smaller numbers you can simply look at the factors or multiples for each number and find the greatest common multiple of them.

For 1, 1, 21, and 15 those factors look like this:

- Factors for 1:
**1** - Factors for 21:
**1**, 3, 7, and 21 - Factors for 15:
**1**, 3, 5, and 15

As you can see when you list out the factors of each number, 1 is the greatest number that 1, 1, 21, and 15 divides into.

## Prime Factors

As the numbers get larger, or you want to compare multiple numbers at the same time to find the GCF, you can see how listing out all of the factors would become too much. To fix this, you can use prime factors.

List out all of the prime factors for each number:

- Prime Factors for 1: 1
- Prime Factors for 21: 3 and 7
- Prime Factors for 15: 3 and 5

Now that we have the list of prime factors, we need to find any which are common for each number.

Since there are no common prime factors between the numbers above, this means the greatest common factor is 1:

GCF = 1

## Find the GCF Using Euclid's Algorithm

The final method for calculating the GCF of 1, 1, 21, and 15 is to use Euclid's algorithm. This is a more complicated way of calculating the greatest common factor and is really only used by GCD calculators.

If you want to learn more about the algorithm and perhaps try it yourself, take a look at the Wikipedia page.

Hopefully you've learned a little math today and understand how to calculate the GCD of numbers. Grab a pencil and paper and give it a try for yourself. (or just use our GCD calculator - we won't tell anyone!)

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