Lumen OHM

Write the following sentence as a proportion:

$\displaystyle {12}$ is to $\displaystyle {15}$ as $\displaystyle {108}$ is to $\displaystyle {135}$

[more..]

\displaystyle

Enter your answer as an equation. Example: y=3x^2+1, 2+x+y=3
Be sure your variables match those in the question