Question

Which of the following are co-primes?
$\left(a\right)8,10$
$\left(b\right)9,10$
$\left(c\right)6,8$
$\left(d\right)15,18$

Answer 1

Co-primes: The common factor of two numbers is $1$.

$\left(a\right)8,10$

Factors of $8$ are $1,2,4,8$

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

common factors are $1,2$

So, these are not co-primes.

$\left(b\right)9,10$

Factors of $9$ are $1,3,9$

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

common factor is only $1$

So, these are co-primes.

$\left(c\right)6,8$

Factors of $6$ are $1,2,3,6$

Factors of $8$ are $1,2,4,8$

common factors are $1,2$

So, these are not co-primes.

$\left(d\right)15,18$

Factors of $15$ are $1,3,5,15$

Factors of $18$ are $1,2,3,6,9,18$

common factors are $1,3$

So, these are not co-primes.

Hence, option $\left(b\right)$ is correct.