The answer would be because if angle BAC= 50 degrees then angle DCE= 50 degrees. These triangles are the same. Each triangle has to equal 180 degrees. We know that this is a 90 degree triangle. 50, 90 is what we have so far and we need angle CDE. 50+90= 140 140+what=180 140+40=180 so the answer is Angle CDE=40 degrees