Question

Given A = {2, 3, 4, 5}, B = {4, 5, 6, 7, 8} and the relation from A to B means, "is a factor of". Represent the relation in roster form.

Also find the number of elements in such a relation.

Answer 1

For the required relation we have to form ordered pairs where $x$ co-ordinate is from set $A$ and is a factor of $y$ co-ordinate which is from Set $B$.
Since $2$ is a factor of $4,6,8$
And $3$ is a factor of $6$
$4$ is a factor of $4,8$
$5$ is a factor of $5$
We have the relation $=\left\{\right(2,4),(2,6),(2,8),(3,6),(4,4),(4,8),(5,5\left)\right\}$.

Thus the number of elements in the given set is $7$.