This page provides a brief overview of *co-prime numbers*.

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What are the factors of `10` and `21`?

- factors of `10` are `1`,`2`,`5`,`10`
- factors of `21` are `1`, `3`, `7`, `21`
- both the above
- both the above

The answer is "both the above".

Factors of `10` are `1`,`2`,`5`,`10`

Factors of `21` are `1`, `3`, `7`, `21`.

Factors of `6` are `1`, `2`, `3`, `6`.

Comparing the factors of `10` and `21`, we find that they do not have any common factors, other than `1`.

Comparing factors of `10` and `6`, we find that they have a common factor `2`.

`10` and `21` are called "**co-prime numbers**".

`10` and `6` are not co-prime numbers, as they share `2` as the common-factor.

Similarly, `6` and `21` are not co-prime numbers, as they share `3` as the common-factor.

Which of the following is a meaning for the prefix "co"?

- perform or work
- together and mutually
- together and mutually

The answer is "together and mutually".

What is the term used to refer to numbers that do not have a common factor?

- Pronunciation : Say the answer once

Spelling: Write the answer once

The answer is "co-prime numbers".

**Co-prime Numbers** : Two or more numbers without any common factor, other than `1`, are called co-prime numbers.

Are `7` and `7` co-prime numbers?

- yes. `7` is a prime number
- no. They have a common factor `7`
- no. They have a common factor `7`

The answer is "no". They have a common factor 7.

Are `21` and `10` co-prime numbers?

- Yes, they do not have a common factor
- Yes, they do not have a common factor
- No, they have `1` as common factor

The answer is "Yes, they do not have a common factor".

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