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

`((C_(H_2))_1)/((C_(H_e))_2)=sqrt((gamma_1RT)/M_1)/sqrt((gamma_2RT)/M_2)=sqrt((gamma_1)/(gamma_2)xx(M_2)/(M_1))`
`=sqrt((7//5)/(5//3)xx4/2)=sqrt(7/5xx3/5xx2)=sqrt(42/25)`