To solve for u, we substitute the first equation in the second one.
[tex]\begin{gathered} 120-15u=30+10u \\ 120-30=10u+15u \\ 25u=90 \\ u=\frac{90}{25}=3.6 \end{gathered}[/tex]Now, we find P.
[tex]\begin{gathered} P=30+10(3.6)=30+36 \\ P=66 \end{gathered}[/tex]