span >A pipe can fill a tank in 14 hours, and B pipe can drain a tank in 21 hours. If a system of N pipes (fill pipes and drain pipes put together) can fill the tank in exactly 7 hours, which of the following are possible values of N (More than one option could be correct)? (1) 21 (2) 13 (3) 18 (4) 24 (5) None of these