Speed of sound in a gas is given by, vs=MγRT
The formula for RMS speed is, vrms=M3RT
It is given that vrms=2vs
⇒vrmsvs=3γ=21⇒3γ=21⇒γ=23
For γ of mixture,
γmix=CV,mixCP,mix=2+n2CV,H+nCV,He2+n2CP,H+nCP,He
Since Hydrogen is a diatomic gas, CV,H=25R,CP,H=27R and Helium is monatomic gas, CV,He=23R,CP,He=25R. Therefore,
γmix=2+n2CV,H+nCV,He2+n2CP,H+nCP,He⇒23=2×25R+n×23R2×27R+n×25R
⇒n=2