Louis and Jack are sharing a pie.  Louis eats $\frac{1}{5}$ of the pie, and Jack eats $\frac{2}{3}$ of the pie.  How much of the pie do Louis and Jack eat altogether?
The denominators $5$ and $3$ have a common multiple of $15$.  We use this to write $\frac{1}{5}\cdot \frac{3}{3}=\frac{3}{15}$ and $\frac{2}{3}\cdot \frac{5}{5}=\frac{10}{15}$.  Then, we can add the fractions by adding the numerators and keeping the denominator.  We have $$\frac{1}{5}+\frac{2}{3}=\frac{3}{15}+\frac{10}{15}=\frac{3+10}{15}=\frac{13}{15}.$$