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
