When you write both fractions in lowest terms, $\displaystyle\frac{{16}}{{20}}\usepackage{cmbright}\Huge\$ simplifies to $\displaystyle\frac{{4}}{{5}}\usepackage{cmbright}\Huge\$ , and $\displaystyle\frac{{15}}{{25}}\usepackage{cmbright}\Huge\$ simplifies to $\displaystyle\frac{{3}}{{5}}\usepackage{cmbright}\Huge\$ . Now both fractions have the same denominator but different numerators, so the two fractions are not equivalent.