In a constant volume gas thermometer, the gas pressures are...

For a constant volume gas thermometer, the temperature $t$ is given by the relation:

$t = \dfrac{P_t - P_0}{P_{100} - P_0} \times 100^{{}^{{}^{\circ}}}\text{C}$

Given values are:

Pressure at ice point, $P_0 = 80 \text{ cm}$ of Hg

Pressure at steam point, $P_{100} = 90 \text{ cm}$ of Hg

Pressure at temperature $t$, $P_t = 100 \text{ cm}$ of Hg

Substituting the values into the formula:

$t = \dfrac{100 - 80}{90 - 80} \times 100^{{}^{{}^{\circ}}}\text{C}$

$t = \dfrac{20}{10} \times 100^{{}^{{}^{\circ}}}\text{C}$

$t = 200^{{}^{{}^{\circ}}}\text{C}$