Calculating side lengths with triangles similarity example when one side is common for both triangles

In this example the side BC is common for both the triangles. Therefore we can you it twice in the similar triangles ratio.

The similar triangles ratio is
k = AB = BC
____ ____
BC DC

k= 20 = 15
___ ____
15 DC

4 = 15
__ __
3 DC

4DC=45
DC=45/4=11 1/4
The length of the side DC is 11 1/4 centimeters.