We have the inequality
[tex]\frac{a}{5}+8<13[/tex]solving for a, we have
[tex]\begin{gathered} \frac{a}{5}+8<13 \\ \frac{a}{5}<13-8 \\ \frac{a}{5}<5 \\ a<5\cdot5 \\ a<25 \end{gathered}[/tex]Then a has to be less than 25. Written the solution in interval form we have:
[tex](-\infty,25)[/tex]