Given that $theta,;φ$ are complementary angles and $tan theta = 0.5$ , what is the value of $tan φ$?

Hi, this is trigonometry and I need help with this. I don’t understand, thanks.

I know how to solve the question using trial and error by plausible values of $tan θ$, and then solving for $tan φ$, but I don’t get how to logically solve it.