Study the following graph and answer the given question.

The graph shows the time (in minutes) taken by the pipes (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 opened at alternate minutes and if pipe P is opened first, then in how many minutes will the tank be full?

From the graph,

Time taken by the pipe P to fill the tank = 15 minutes

Time taken by the pipe Q to fill the tank = 30 minutes

Let the Volume of the tank = 30 units (LCM of 15 and 30)

Volume filled by pipe P in 1 min = 2 units

Volume filled by pipe Q in 1 min = 1 unit

If they are opened at alternate minutes,

Volume filled by pipes P and Q in 2 min = 2 + 1 = 3 units

Time required to fill the tank = $$\frac{30}{3}\times2$$ = 20 minutes

Hence, the correct answer is Option D