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 kg·s⁻¹ with a temperature of 100 °C and leaves at 40 °C. Water enters the inner tube at a mass flow rate of 1 kg·s⁻¹ with a temperature of 20 °C and leaves at 80 °C. Use specific heats of oil and water as 2089 J·kg⁻¹·K⁻¹ and 4178 J·kg⁻¹·K⁻¹, respectively. There is no phase change in both the streams. Under steady-state conditions, the number of transfer units (NTU) is _______ (in integer).