Answer
$1500$
Work Step by Step
Using the formula from the example, if the number of new movies bought is $n_1$ and the number of old movies bought is $n_2$, then $n_1=\frac{12000-3n_2}{5}$.
Hence here $n_1=\frac{12000-3(1500)}{5}=\frac{12000-4500}{5}=\frac{7500}{5}=1500$.