You can write both fractions with a common denominator, 70. Rewrite $\displaystyle\frac{{15}}{{35}}\usepackage{cmbright}\Huge\$ as $\displaystyle\frac{{30}}{{70}}\usepackage{cmbright}\Huge\$ . Now both fractions have the same denominator but different numerators, so the two fractions are not equivalent. The correct answer is $\displaystyle\frac{{6}}{{14}}\usepackage{cmbright}\Huge\$ .