The least common multiple of 5 and 15 is 15, so rewrite the fraction $\displaystyle\frac{{2}}{{5}}\usepackage{cmbright}\Huge\$ with a denominator of 15. $\displaystyle\frac{{2}}{{5}}\cdot\frac{{3}}{{3}}=\frac{{6}}{{15}}\usepackage{cmbright}\Huge\$