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

(R+C)-R=C

(.66')-(.4)=C

.266'=C

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

Ans=15 hours