Using the relation $\frac{T - \text{LFP}}{\text{UFP} - \text{LFP}} = \text{constant}$, we write $\frac{T - 0}{100 - 0} = \frac{T - 40}{80 - 40}$. This simplifies to $\frac{T}{100} = \frac{T - 40}{40}$, which gives $40T = 100T - 4000$ or $60T = 4000$, hence $T = \frac{200}{3}\,^\circ\text{C}$.

The change in temperature on the Celsius scale is equal to the change in temperature on the Kelvin scale because the size of one degree Celsius is equal to one Kelvin: \(\Delta T_{text{C}} = \Delta T_{\text{K}} = 2\text{ K}\).

According to the Zeroth Law of Thermodynamics, temperature is the physical quantity that determines if systems are in thermal equilibrium with each other.

Using the relation $\frac{T - \text{LFP}}{\text{UFP} - \text{LFP}} = \frac{C - 0}{100 - 0}$, we substitute $T = C$ for correct reading. This gives $\frac{C - 40}{80 - 40} = \frac{C}{100}$, which simplifies to $C = \frac{200}{3} ^\circ\text{C}$.