[tex] \frac{\big{15}}{\big{y}} = \frac{\big{40}}{\big{12}}\ \ \ \Rightarrow\ \ \ the\ domain:\ \ \ D=R-\{0\} \\\\ \frac{\big{15}}{\big{y}} = \frac{\big{4\cdot10}}{\big{4\cdot4}} \\\\ \frac{\big{15}}{\big{y}} = \frac{\big{10}}{\big{3}} \ \ \ \Leftrightarrow\ \ \ 15\cdot3=10\cdot y\\\\45=10y\ /:10\\\\4.5=y\\\\4.5\in D\\\\Ans.\ \ y=4.5[/tex]
