George can paint at a rate of 1/30th of a house per hour.
Combine their efforts and they can paint 1/20 + 1/30 = 1/12th of a house in an hour.
So if their rate is 1/12th of a house per hour it will take 12 hours to paint an entire house.
Thanks to Marilyn Vos Savant for this one.
Michael Shackleford, A.S.A.