Welcome to MHF zerodollarbaby,
First lets figure out how fast ray works in rooms per hour.
Ray by himself can paint 4 rooms in 10 hours.
R = 4 rooms / 10 hours= 0.4 rooms/hour
Then lets figure out how fast ray and chris work together
R+C= 4 rooms/ 6 hours = .66' rooms/hour
Now lets subtract Ray's work rate from Ray and Chris' work rate
Chris paints at a rate of .266' rooms/hour
now divide the number of rooms by his work rate to find how many hours it would take him alone.
4 rooms / (.266' rooms/hour) ;rooms cancel and the unit becomes hours