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.