$($a$)$ Find the average time required for an oxygen molecule to diffuse through a $0\mathrm{.}200-$mm$-$thick tear layer on the cornea. $($b$)$ How much time is required to diffuse $0\mathrm{.}500$ cm$3$ of oxygen to the cornea if its surface area is $1\mathrm{.}00$cm$2?$