A carpenter is working on a deck. He hires two assistants who work 60% as fast as he works. If all three work together on the deck, they should finish in what fraction of the time that he would take working alone?
Carpenter rate = 1 job / 1
assistant rates = 1 job / (6/10)
All three 1 job / ( 1 + 6/10 + 6/10) = 1 / 2.2 = 5/11