A wall consists of a $80$ mm layer of brick $($with thermalconductivitykj$=0\mathrm{.}7$ W$/$mK$)$ followed by a $40$ mm layer of gypsum plaster $($k$2=0\mathrm{.}065$W$/$mK$).$To reduce heat loss or gain through the wall by $50\%,$ what thickness of loosely packed rock$-$wool insulation $($k$3=0\mathrm{.}055$W$/$mK$)$ should be added?