# How to relate cross sectional area to total surface area?

This problem particularly deals with application of Fourier's law of heat conduction.At some point we are to calculate volume to total surface area ratio.However with length of the slab(given as ratio) and thermal conductivity being asked as a ratio it is very difficult for me to relate total surface area to cross sectional area.Somebody suggested me that for a rectangular slab they are supposed to be equal.However I was not convinced with the explanation.Please help me with this.