Question 857612
Let's call a day an hour, just for clarity.  Doesn't affect the equation.
M = J = R (same rate for all of them)
It takes 9 man-hours * 2 (technically, "man-days", but I don't know if that's a real term) to paint a house.  18 man-hours total.
When Ron joins them, it still takes 18 man-hours to paint the house, but now it's divided by three people.  18 / 3 = 6
Answer: 6 days