The temperature of a given mass of a gas is increased from 19°C to 21 °C at constant pressure. The volume V of the gas is increased 

The temperature of a given mass of a gas is increased from 19°C to 21°C at constant pressure. The volume V of the gas is increased by 2/273.15 of its volume at 0°C.

Initial temperature is 19°C or 292 K and final temperature is 20°C or 293 K. 

Pressure is constant, therefore $\mathrm{V}\infty \mathrm{T}...(\mathrm{charle}\text{'}\mathrm{s} \mathrm{law})$

$$\begin{gathered} \frac{{\mathrm{V}}_{{19}^0\mathrm{C}}}{292}=\frac{{\mathrm{V}}_{{20}^0\mathrm{C}}}{293} \\ {\mathrm{V}}_{{20}^0\mathrm{C}}=\frac{293\times {\mathrm{V}}_{{19}^0\mathrm{C}}}{292} \mathrm{or} \frac{20 {\mathrm{V}}_{{19}^0\mathrm{C}}}{19}\mathrm{or} \frac{20 \mathrm{V}}{19} \end{gathered}$$