To find out the new temperature, we can use Charle's law.
Charle's law says "at a constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature".
V α T
Where V is the volume and T is the temperature in Kelvin of the gas. We can use this for two situations as,
V₁/T₁ = V₂/T₂
V₁ = 50 L
T₁ = 40 K
V₂ = 30 L
T₂ = ?
By applying the formula,
50 L / 40 K = 30 L / T₂
T₂ = (30 L / 50 L) x 40 K
T₂ = 24 K
Hence, the new temperature of the gas is 24 K.
Assumption made - The gas has an ideal gas behavior.
Pressure of the gas remains constant.