
Using the equation of conduction,
dtdQ=lKAΔT
Now the total area of the box will be,
A=2(0.6×0.5+0.5×0.2+0.2×0.6)
A=2(0.3+0.1+0.12)
A=2(0.52)=1.04m2
Therefore,
dtdQ=lKAΔT=0.010.05×1.04×40=208Js−1
This heat will melt the ice. Therefore,
dtdQ=dtdmL⇒208=dtdm×3.4×105⇒dtdm=61×10−5kgs−1