[tex]\frac{CD}{CA}=\frac{CE}{CB}[/tex]
To find CD, you solve the equation above for CD:
As CD is beeing divided into CA, you multiply both sides of the equation by CA:
[tex]\begin{gathered} CA\cdot\frac{CD}{CA}=CA\cdot\frac{CE}{CB} \\ \\ CD=\frac{CA\cdot CE}{CB} \end{gathered}[/tex]
Then, for the given values:
[tex]CB=CE+EB[/tex][tex]CD=\frac{CA\cdot CE}{CE+EB}[/tex]
[tex]CD=\frac{18\cdot20}{20+25}=\frac{360}{45}=8[/tex]
