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.