Saturated vapor at $200\: ^{\circ}C$ condenses to saturated liquid at the rate of $150$ $\text{kg/s}$ on the shell side of a heat exchanger (enthalpy of condensation $h_{fg} = 2400$ $\text{kJ/kg}$). A fluid with $C_{p} = 4\: kJ \cdot kg^{-1} \cdot K^{-1}$ enters at $100 \:^{\circ}C$ on the tube side. If the effectiveness of the heat exchanger is $0.9$, then the mass flow rate of the fluid in the tube side is ____________ $\text{kg/s}$ (in integer).