A professional janitor can clean a certain high school in (500-20) minutes, working at a constant rate. A student sentenced to detention can clean that same high school in 20 hours, also working at a constant rate. If the student is paid $7 total per hour and the janitor is paid $21 per hour, how much more would it cost the school to pay the janitor to do the job himself than it would to pay the student and the janitor to do the job together?
Pick:  A) -$56  B) -$6  C) $0  D) $6  E) $8
A professional janitor can clean a certain high school in (500-20)/60 or 8 hours   So (applying rule #1), the janitor can clean 1/8 of the school in ONE HOUR   A student sentenced to detention can clean that same high school in 20 hours   So (applying rule #1), the student can clean 1/20 of the school in ONE HOUR   So, COMBINED, the student and janitor can clean (1/8 + 1/20) of the school in ONE HOUR   1/8 + 1/20 = 5/40 + 2/40 = 7/40   So, in ONE HOUR they can clean 7/40 of the school.   Applying rule #2, it will takethem40/7 hoursto clean the ENTIRE school.   The janitor earns $21/hour and the student earns $7/hour, so their combined rate is$28/hour.   Theircombined wages= (pay rate)(time) = ($28/hour)(40/7 hours) =$160   Working ALON$8, the janitor takes 8 hours and earns $21/hour   So, working alone, the janitor's earnings = (pay rate)(time) = ($21/hour)(8 hours) =$168   $168-$160= $8, so the answer is $8 168- 160= 
<gadget id="calculator"> 168- 160</gadget>
<output>8</output>
$8, so the answer is $8
<result>E</result>
A professional janitor can clean a certain high school in (500-20) minutes, working at a constant rate. A student sentenced to detention can clean that same high school in 20 hours, also working at a constant rate. If the student is paid $7 total per hour and the janitor is paid $21 per hour, how much more would it cost the school to pay the janitor to do the job himself than it would to pay the student and the janitor to do the job together?