$3\mathrm{.}2$ Assuming unit formation width, determine the average porosity for the following system when ${}_a=0\mathrm{.}20,{}_b=0\mathrm{.}11,{}_c=0\mathrm{.}29,L_1=0\mathrm{.}35L,$ and $h_a=h_b=0\mathrm{.}5h_c.$