Two angles are plotted on one side of line MK: angle MKC = 40 degrees and angle MKD = 60 degrees.

Two angles are plotted on one side of line MK: angle MKC = 40 degrees and angle MKD = 60 degrees. What is the CKD angle?

Angle MKD and angle MKC have a common side and a common vertex. Basically MKD is composed of MKC and CKD. Then:
angle MKD = angle MKC + angle CKD.
The CKD angle will be equal to the difference between the MKD angle and the MKC angle. Then:
angle CKD = angle MKD – angle MKC;
angle CKD = 60 degrees – 40 degrees;
angle CKD = 20 degrees.
Answer: angle CKD = 20 degrees.