Aptitude - Pipes and Cistern

Exercise :: Pipes and Cistern - Data Sufficiency 2

Each of the questions given below consists of a question followed by three statements. You have to study the question and the statements and decide which of the statement(s) is/are necessary to answer the question.

If both the pipes are opened, how many hours will be taken to fill the tank?
I.
The capacity of the tank is 400 litres.
II.
The pipe A fills the tank in 4 hours.
III.
The pipe B fills the tank in 6 hours.