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.