Time unit is HOURs and one job is GRADE ALL THE EXAMS, and you can just call this 1 JOB.
Graders RATE as JOBS/HOURS
Teacher&Asst
Teacher
Assistant
Study that table of data and be sure you understand how to form it. See that the time for the teacher to do ONE job is assigned to . Any variable could have been chosen.
Rate of people combined to do the same task are to be simply summed.
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You can very likely do the rest of this. Say if otherwise and if you post another on the site for this same question, include the question number #1030345.