Study the following graph and answer the given questions. The graph shows the time(in minutes) takes by the pipe (A,B), (C, D), (E, F), (G, H) and (P, Q) to fill a tank. Two pipes P and Q are inlet pipes. If they are are opened an alternate minutes and if pipe P is opened first, then in how many minutes will take be full ?

18 minutes 20 minutes 16 minutes  30 minutes

20 minutes