so hmmm check the picture below
so, we're looking for dr/dt then at 4:00pm or 4 hours later
now, keep in mind that, the distance "x", is not changing, is constant whilst "y" and "r" are moving, that simply means when taking the derivative, that goes to 0
[tex]\bf r^2=x^2+y^2\implies 2r\cfrac{dr}{dt}=0+2y\cfrac{dy}{dt}\implies \cfrac{dr}{dt}=\cfrac{y\frac{dy}{dt}}{r}\quad
\begin{cases}
\cfrac{dy}{dt}=30\\
r=130\\
y=120
\end{cases}
\\\\\\
\cfrac{dr}{dt}=\cfrac{120\cdot 30}{130}[/tex]
