A thermocouple is used to measure the temperature, when the measuring part is immersed in water at its triple point the voltmeter reading was 100 mV and when it immersed in water at its steam temperature the voltmeter reading was 500 mV. When the thermocouple is used to measure the temperature of a body with unknown temperature, T, as shown in the figure, the voltmeter reading was 618 mv. Find the unknown temperature in Celsius.