Question 121005
Let x=amount of time it takes them to paint a house working together
In 20 hrs, Joe can paint 1 house so Joe paints at the rate of 1/20 house per hour 

Ross paints at the rate of 1/60 house per hour

Together, they can paint at the rate of 1/20+1/60 house per hour or:
1/20+1/60=3/60+1/60=4/60=1/15 house per hour

So our equation to solve is:
(1/15)x=1 (house) multiply each side by 15
x=15 hours

Hope this helps---ptaylor