Assuming that traffic on a highway obeys the Greenshields model, the speed of a shockwave between two traffic streams $\text{(P)}$ and $\text{(Q)}$ as shown in the schematic is _________ $\text{kmph}$. (in integer)

$$\overset{\text{Direction of traffic}}{\longrightarrow}$$  $$\begin{array}{c|c} \hline \text{(P) Flow = 1200 vehicles/hour} & \text{(Q) Flow = 1800 vehicles/hour} \\ \text{Speed = 60 kmph} & \text{Speed = 30 kmph} \\\hline \end{array}$$