I have 2 house cleaners, they clean this house in 2 hours working together, if I add a third cleaner how much time would it take to complete?

Question

I have 2 house cleaners, they clean this house in 2 hours working together, if I add a third cleaner how much time would it take to complete?

Answer 1

in one hour they each do 1/2 a house

all three do 3/2 a house in one hour

and do one house together in 1/(3/2)=2/3 of an hour