From the numbers 2,3,4,5,6,7,8,9 how many pairs of co- primes can be formed and how ?

Two numbers are coprime if their highest common factor (or greatest common divisor if you must) is 11.

You can have the set of positive integers which are coprime to a given number.

Given numbers are,
2,3,4,5,6,7,8,9

Look how many prime numbers are there:
2,3,5,7 are prime

A pair of co-prime can be formed with each of these prime numbers are:
2,3
2,5
2,7
2,9

3,4
3,5
​​​​3,7
3,8

4,5
4,7
4,9

5,6
5,7
5,8
5,9

6,7

7,8
7,9

8,9

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