Answer
$$p = 0.0422 \space atm$$
Work Step by Step
Perfect gas law: $pV = nRT$
Molar mass (Ne): $20.18 \space g/mol$
$1 \space g = 1000 \space mg$
$R = 8.20574 \times 10^{−2} \space dm^3 \space atm \space K^{−1} \space mol^{−1}$
$$n = \frac m {mm} = \frac{255 \space mg}{20.18 \space g/mol} \times \frac{1\space g}{1000 \space mg} = 0.0126363 \space mol$$
$$pV = nRT \longrightarrow p = \frac{nRT}{V}$$ $$p = \frac{( 0.0126363 \space mol)(8.20574) \times 10^{−2} \space dm^3 \space atm \space K^{−1} \space mol^{−1})(122 \space K)}{3.00 \space dm^3}$$ $$p = 0.0422 \space atm$$
