\(v _{ rms } \propto \sqrt{ T }\)
\(\frac{\left( v _{ rms }\right)_{2}}{\left( v _{ rms }\right)_{1}}=\sqrt{\frac{ T _{2}}{ T _{1}}}\)
\(=\sqrt{\frac{400}{300}}\)
\(=\frac{2}{\sqrt{3}}\)
\(\left( v _{ rms }\right)_{2}=\frac{2}{\sqrt{3}}\left( v _{ rms }\right)_{1}\)
\(=\frac{2}{\sqrt{3}} \times 200\)
\(\left( v _{ rms }\right)_{2}=\frac{400}{\sqrt{3}} m / s\)
\(=400\)