I3.2 Let $P(x, b)$ be a point on the straight line $x+y=30$ such that slope of $O P=a$ ( $O$ is the origin).
Determine $b$. (Reference: 1994 FI1.4)
$$
\begin{array}{l}
x+b=30 \\
\Rightarrow x=30-b \\
m_{O P}=\frac{b}{30-b}=4 \\
\Rightarrow b=120-4 b \\
\Rightarrow b=24
\end{array}
$$