Awater barrel has two pipes for filling (a and b) and one pipe for draining (c). if only pipe a is open, the empty barrel will fill in 1 hour. if only pipe b is open, the empty barrel will fill in 50 minutes. if only pipe c is left open, the full barrel will empty in 40 minutes. suppose that all three pipes are left open when the barrel is initially empty. how many minutes will it take to fill? round your answer to the nearest minute.