SOLUTION: John can paint a room three times as fast as Mary. Together, they can paint the same room in 1.5 hours. How long does it take each one working alone to paint that room?

John works as effectively as three workers equivalent to Mary work.


So, when John and Mary work together,  they work as effectively as 4 workers like Mary work.


Thus you can reformulate the given part in this way:



    4 workers equivalent to Mary can paint the room in 1.5 hours.



Then it is clear that Mary alone can do this job in 4*1.5 = 6 hours.


Since John can do it in 3 times faster, he needs only 2 hours.


Answer.  John needs 2 hours to complete the job working alone.