Answer: 1/15 of a classroom per minute
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Work Shown:
1/3 of a classroom = 5 minutes
3*(1/3 of a classroom) = 3*5 minutes
1 classroom = 15 minutes
It takes the janitor 15 minutes to mop a full classroom.
The rate is expressed in units of classrooms per minute. This means we'll need to divide both sides by 15 to get
1 classroom = 15 minutes
1/15 classroom = 15/15 minutes
1/15 of a classroom = 1 minute
The janitor's rate is 1/15 of a classroom per minute.