Make sure to set up the proportion correctly. This is the solution to $\displaystyle\frac{{1}}{{2}}=\frac{{30}}{{n}}\usepackage{cmbright}\Huge\$ . Each ratio should compare things in the same order, such as pet owners to all students. The correct proportion would have been $\displaystyle\frac{{1}}{{2}}=\frac{{n}}{{30}}\usepackage{cmbright}\Huge\$ .