25. As shown in the figure below, circles $C_{1}$ and $C_{2}$ of radius 360 are tangent to each other, and both tangent to straight line $\ell$. If circle $C_{3}$ is tangent to $C_{1}, C_{2}$ and $\ell$, and circle $C_{4}$ is tangent to $C_{1}, C_{3}$ and $\ell$, find the radius of $C_{4}$.
25. Answer: 40 .
Let $R$ be the radius of $C_{3}$. Then
$$
(360-R)^{2}+360^{2}=(360+R)^{2} \Rightarrow R=90 .
$$

Let $r$ be the radius of $C_{4}$. Then
$$
\sqrt{(360+r)^{2}-(360-r)^{2}}+\sqrt{(90+r)^{2}-(90-r)^{2}}=360 \Rightarrow r=40
$$