CONCEPT: The concept of Time and Work(Rates) has been used in the question
Let the total work be LCM(20,15) = 60 units
Their efficiencies are 60/20 and 60/15 = 3 and 4 units respectively
Together 3 + 4 = 7 units/ rooms are cleaned in unit time(an hour)
Hence, working together, the time to clean 1 room is 1/7 hr.
Hence to complete 30 units they would take 30 / 7 hours (OPTION D)