Apply Grahams law of effusion.
rate(r) of effusion/diffusion is proportional to 1/ root [density(d) ]
hecne r1/r2 = root( d2/d1 )
=> r1/r2 = root ( M2/M1 ) since [2. vapour density(d)=molecular mass(M) ]
now put rate=Volume/time=V/t
all sums of this type are solved by this concept.
Hope it gets cleared now.