Question 755550
More than half the solution process is here:

 8 students 20 minutes to wash 3 cars

By intuition for the ratio, 8 students need 20/3 minutes to wash 1 car.  
The job seems to be, "wash one car", and we want the rate for 1 student.
One student will need 8 times longer to wash 1 car, so:
1 student needs {{{8*20/3}}} minutes to do the job of washing 1 car. 


This one-student job rate is then {{{1/((8*20)/3)=highlight(3/(240))}}} jobs per minute. 


Can you do the rest of the solution?  {when the student work at the same time, their rates, each the same, are additive.}