If the temperature of a gas is increased from 27°C to 927°C, the root mean square speed of its molecules 

The root mean square speed is given by 

               V_rms=$\sqrt{\frac{3\mathrm{kT}}{\mathrm{m}}}$

 i.e. v_rms $\propto$T. Initial temperature T₁ = 27 + 273 = 300 K. Final temperature T₂ = 927 + 273 = 1200 K. Since temperature is increased by 4 times, the speed is doubled.