Let the number of reptiles be x.
If there are twice as many mammals as reptiles, then the number of mammals will be:
[tex]x\times2=2x[/tex]The total number of pets in the store is 39. Therefore, we have that:
[tex]x+2x=39[/tex]Solving, we have:
[tex]\begin{gathered} 3x=39 \\ x=\frac{39}{3} \\ x=13 \end{gathered}[/tex]The correct option is OPTION C.