A cistern can be filled by two pipes A and B in 4 hours and 6 hours respectively. When full, the tank can be emptied by a third pipe C in 8 hours. If all pipes are turned on at the same time, the cistern will be full in
\( \frac{40}{7} \) hours
\( \frac{24}{7} \) hours
\( \frac{16}{7} \) hours
\( \frac{31}{7} \)hours
In which group of the modern periodic table are halogens placed?