I FOUND YOUR COMPLETE QUESTION IN OTHER SOURCES. SEE ATTACHED IMAGE.
For this case, what we are going to do is study each part separately. Left diagram: In this diagram you show three quarters of the hours. We have then: (3/4) * (12) = 9 Answer: You teach 9 hours. Right diagram: In this diagram you teach half of the hours. We have then: (2/4) * (12) = 6 Answer: You teach 6 hours