If butyl rubber gloves (0.04 cm thick) are used, what is the breakthrough time (tb), i.e., how long could the gloves be used before methylene chloride reaches the hand?

–diffusion coefficient in butyl rubber:

D = 110 x10-8 cm2/s

When we solve this problem, we use this eq. Tb=l^(2)/6D

How can I make this equation???