A gas thermometer determines temperature from the variation of...

For a constant volume gas thermometer, the temperature $T$ is directly proportional to the pressure $P$.

$T = T_{tr} \times \dfrac{P}{P_{tr}}$

Given that the pressure at the melting point of lead is $P = 2.20 P_{tr}$ and the temperature of the triple point of water is $T_{tr} = 273.16\text{ K}$.

Substituting the values:

$T = 273.16 \times \dfrac{2.20 P_{tr}}{P_{tr}}$

$T = 273.16 \times 2.20 = 600.952\text{ K}$

Rounding off to the nearest integer, the melting point of lead is $601\text{ K}$.