Problem
Show that the fraction $\frac{15n+4}{10n+3}$ is irreducible for any $n \in \mathbb N$
Source
This problem was given during a national high-school (equivalent) algebra test in Norway.
My attempt
To be honest, the only thing I could think to try was to see whether the numerator minus the denominator was 1, which would have concluded the proof. Since it was not, I don't know where to go from here.