Correct. $\displaystyle\frac{{12}}{{16}}=\frac{{3}}{{4}}=\frac{{30}}{{40}}\usepackage{cmbright}\Huge\$ , so $\displaystyle\frac{{30}}{{40}}=\frac{{{4}{z}+{6}}}{{40}}\usepackage{cmbright}\Huge\$ . This means $\displaystyle{30}={4}{z}+{6}\usepackage{cmbright}\Huge\$ , so $\displaystyle{z}={6}\usepackage{cmbright}\Huge\$ .