If four people can clean a park in 9 hours how long would it take for 6 people to clean it

Here's how I untangle problems like this one.
Each step comes from the step before it.

Given . . . 4 people do the job in 9 hours.

-- 4 people do (1/9) of the job in 1 hour.

-- 1 person does (1/9) / 4 = (1/36) of the job in 1 hour.

-- 6 people do (6) x (1/36)  = (6/36) = (1/6) of the job in 1 hour.

--  (1 job) / (1/6 job per hour)  =  6 hours.



That's one way.
Here's another:

-- The more people work on the job, the less time it takes.

-- This tells us that the time is inversely proportional to the
number of people.

-- If the number of people changes, then the time changes by
the same factor, but in the other direction.

-- The number of people was multiplied by 3/2 ... from 4 to 6.

-- So the time was multiplied by 2/3 . . . 2/3 x 9 = 6 hours.

This method may be too squishy for high schoolers to try and walk on.

It would take 13 1/2 hours because if you cross multiple and divide. 9x6=54 and 54/4=13 1/2