The rate of diffusion is inversely proportional to square root of molecular weight. The molecular weight of B is 18g∕mol. Rate is directly proportional to distance. ∴‌‌‌
rA
rB
=‌
dA
dB
=√‌
MB
MA
where, r,d and M are rate, distance travelled and molar mass respectively. dA‌‌=40m,dB=100−40=60m ‌