Question

The number of coprime pairs that can be formed by the factors of $80$ is

• A. $4$
• B. $8$
• C. $13$
• D. $24$

Answer 1

The correct option is C $13$

Factors of $80$ are
$\{1,2,4,5,8,10,16,20,40,80\}$
Here, every number is coprime with $1$
$\Rightarrow \left\{\right(1,2),(1,4),...,(1,80\left)\right\}$

Other coprime pairs are $\left\{\right(2,5),(4,5),(5,8),(5,16\left)\right\}$
So, total pairs are $9+4=13$