Question 563720
Three people clean the office in 2 hours means that it takes 6 person-hours of work to clean the office (3 persons times 2 hours = 6 person-hours).
.
So 4 persons will need to work 6 person-hours to clean the office. 
.
To find the number of hours 4 persons need to work to complete 6 person-hours, divide 4 persons into 6 person-hours and you get the answer of 1.5 hours (4 into 6 = 1.5).
.
Or if you prefer, since a half hour is 30 minutes, 1.5 hours is the same as 1 hour and 30 minutes for the 4 persons to completely clean the office.
.
Hope this helps you to understand the problem.
.