According to Graham's law of diffusion, the ratio of the diffusion rates of two gases is the inverse of the square root of the ratio of their molar masses. Therefore:
Rate of A / Rate of B = √(Molar mass of B / Molar mass of A)
Rate of A / Rate of B = √(64 g/mol / 32 g/mol) = √2 = 1.41
So, the ratio of their diffusion rates is approximately 2:1.