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 = 800 mL
T₁ = -55.0 ⁰C = 218 K
V₂ = ?
T₂ = 25.0 ⁰C = 298 K
By applying the formula,
800 mL / 218K = V₂ / 298 K
V₂ = (800 mL / 218 K) x 298 K
V₂ = 1093.58 mL
Hence, the volume of the gas at 25.0 ⁰C is 1093.58 mL