Ernie can paint a house in just 2 hours. Izzy can paint a house in 5 hours (because she takes breaks to read and sip lemonade ). Working together, how long will it take them to paint 2 houses?
Ernie paints 1/2 a house per hour
Lizzie paints 1/5 of a house per hour
Together they paint 1/5 + 1/2 = 7/10 of a house in an hour
So that is 10/7 hours per house, and that's 20/7 hours for two houses