b
${v_{rms}} = \sqrt {\frac{{3RT}}{M}} $
$\Rightarrow \frac{{{{({v_{rms}})}_{{O_2}}}}}{{{{({v_{rms}})}_{{H_2}}}}} = \sqrt {\frac{{{T_O}_2}}{{{T_{{H_2}}}}}} $
$ \Rightarrow \frac{{{{({v_{rms}})}_{{O_2}}}}}{{{{({v_{rms}})}_{{H_2}}}}} = \sqrt {\frac{{900}}{{300}} \times \frac{2}{{32}}} = \frac{{\sqrt 3 }}{4}$
${({v_{rms}})_{{O_2}}} = 1930 \times \frac{{\sqrt 3 }}{4}$$=836\,m/sec$