At 400K, the root mean square (RMS) speed of a gas X (molecular weight=40) is equal to the most probable speed of gas Y at 60K. The molecular weight of the gas Y is _______.

Vrms(X gas)(400K)=root mean square speed of gas X at 400K
Vmp(Y gas)(60K)=most probable speed of gas Y at 60K
Vrms(X gas)(400K)=Vmp(Y gas)(60K)
M.W. (X gas)=40; M.W. (Y gas)=x
√(3RT₁/M₁)=√(2RT₂/M₂)
√(3 × 400/40) = √(2 × 60/x)
√30 = √(120/x)
30 = 120/x
x = 120/30
x = 4 g/mol