A gear train shown in the figure consists of gears $P, Q, R,$ and $S$. Gear $Q$ and gear $R$ are mounted on the same shaft. All the gears are mounted on parallel shafts and the number of teeth of $P, Q, R$ and $S$ are $24, 45, 30,$ and $80$, respectively. Gear $P$ is rotating at $400$ rpm. The speed (in rpm) of the gear $S$ is __________.