A perfectly insulated, concentric tube countercurrent heat exchanger is used to cool lubricating oil using water as a coolant (see figure below). Oil enters the outer annulus at a mass flow rate of $2 \mathrm{~kg} \cdot \mathrm{s}^{-1}$ with a temperature of $100^{\circ} \mathrm{C}$ and leaves at $40^{\circ} \mathrm{C}$. Water enters the inner tube at a mass flow rate of $1 \mathrm{~kg}$. $\mathrm{s}^{-1}$ with a temperature and $4178 \mathrm{~J}_{\mathrm{kg}} \mathrm{kg}^{-1} \mathrm{~K}^{-1}$, respectively. There is no phase change in both the streams. Under steady-state conditions, the number of transfer units $\text{(NTU)}$ is_____________(in integer).
