Question

your boss estimates it take 42 person - hours to rearrange the floor plan for the produce department at a local grocery store. Your boss would like to complete the job in 9 hours. How many workers are necessary to complete the job in this amount of time?

Answer 1

It is important to read the question very carefully. The trick lies in the way the question is written. Once it is understood, then the problem is easy.
Number of persons required to rearrange the floor plans in 1 hour = 42
Number of persons required to rearrange the floor plans in 9 hours = 42 * 9
                                                                                                             = 378 hours
So it will take 378 hours by 9 persons to rearrange the floor. I hope this is the answer you were looking for and the procedure is also clear to you. Once you understand the procedure, you can attempt such problems on your own in future.

Answer 2

9 hours* (42 people/ 1 hour)= 378 people.
(Note that the unit cancels out so you get the answer)

378 workers are necessary to complete the job in that amount of time~