Barrier food packaging is often made from multilayer composite structures where a very thin layer of an expensive high barrier polymer is sandwiched between layers of a less expensive polymer, which provides mechanical strength to the composite structure. Derive a formula to calculate the steady state permeability of a three layer flat sheet composite film. The final result should give the composite permeability, Pcomp, in terms of the permeabilities and thicknesses of each layer. Use this result to calculate oxygen permeability at $23\backslash$deg C of a three layer composite structure consisting of an inner and outer layer of $15$ mils of poly$($ethylene$)($PPE$=110$ cm$3($STP$)$ mil$/(100$ in$2$ day atm$))$ jacketing a $3$ mil barrier layer of poly$($ethylene$-$co$-$vinyl alcohol$),$ EVOH $(70\%$VOH$),($PEVOH$=0\mathrm{.}017$ cm$3($STP$)$ mil$/100$ in$2$ day atm$)).$ Note that $1$ mil $=0\mathrm{.}001$ inch.