Question

Out of $15,16,21$ and $28,$ find out all the pairs of co-prime numbers.

Answer 1

Step: Find all possible pairs of co-prime numbers
$15$ and $16,15$ and $21,15$ and $28,16$ and $28,$ and $21$ and $28$ will be possible pairs.

$H.C.F.$ of two co-prime numbers is always $1.$
$H.C.F.$ of $15$ and $16$ using division method,

Here, the last division is $1.$
Thus, the $H.C.F.$ of $15$ and $16$ is $1.$
$H.C.F.$ of $15$ and $21$ using division method,

Here, the last division is $3$.
Thus, the $H.C.F.$ of $15$ and $21$ is $3$.

$H.C.F$. of $15$ and $28$ using division method,

Here, the last division is $1.$
Thus, the $H.C.F.$ of $15$ and $28$ is $1$.

$H.C.F.$ of $16$ and $21$ using division method,

Here, the last division is $1$.
Thus, the $H.C.F.$ of $16$ and $21$ is $1$.

$H.C.F.$ of $16$ and $28$ using division method,

Here, the division is $4$.
Thus, the $H.C.F.$ of $16$ and $28$ is $4$.

$H.C.F.$ of $21$ and $28$ using division method,

Here, the last division is $7.$
Thus, the $H.C.F.$ of $21$ and $28$ is $7$.
Hence, $15$ and $16,15$ and $28,16$ and $21$ are co-prime numbers.