Pipe A can fill a tank in 16 minutes and pipe B can empty it in 24 minutes. If both the pipes are opened together after how many minutes should pipe B be closed, so that the tank is filled in 30 minutes?
Three men do as much work as 4 boys do and the 4 boys can together complete work in 21 days. In how many days will 9 men and 2 boys complete the same work?