Margot can clean her room alone in 50 min If Jeremy helps he

Margot can clean her room alone in 50 min. If Jeremy helps her, it takes 30 min. How long would it take Jeremy alone to clean the room?

Solution

Margot can clean her room alone in 50 min. Thus she can clean 1/50 of the room in 1 minute. If Jeremy helps her, it takes 30 minutes for both to clean the room. Therefore, Jeremy, alone , can clean 1/30 - 1/50 = 2/150 = 1/75 of room in 1 minute. Therefore Jeremy alone can clean the room in 75 minutes.