The ratio of the velocity of sound in Hydrogen gas `(gamma=7/5)` to that in Helium gas `(gamma=5/3)` at the same temperature is `sqrt(21/3)`.

Explanation

`v_(sound)=sqrt((gammaRT)/(M)), (v_(H_(2)))/(v_(He))=sqrt((gamma_(H_(2))//M_(H_(2)))/gamma_(H_(2)//M_(H_(e))))=sqrt(42)/25`