What is the length of cd ? in this diagram abc edc ?

CD = x = 4 units

Explanation:
We are given that the two triangles are similar. This means that we can set the similarity proportionality as follows:


W e have:
BC = 20 - x
AC = 20
CD = x
CE = 5

Substitute in the above proportionality and solve for x as follows:


4x = 20 - x
5x = 20
x = 4

Based on he above:
CD = 4 units
BC = 20 - x = 20 - 4 = 16 units

Hope this helps :)