Mass transfer question

Please include diagram.

It has been reported that there was an accident near a gas station, in which, a tanker full of gasoline was broken and all its gasoline were spread all of a sudden on top of the soil close to the gas station. Assume that the top layer of the soil immediately was saturated with gasoline with molar concentration of Cas, where A is gasoline. The diffusion coefficient of gasoline in soil is DAB, where B is the soil. a) Find a governing equation in form of a differential equation along with all boundary conditions necessary so that one can solve it to find the concentration of gasoline in soil as a function of time and distance. Solution is not necessary. (20 points) b) Optional (10 extra bonus points if solved): Solve the differential equation in part (a) and apply boundary conditions to find an expression for the gasoline concentration in the soil in terms of time and distance. Also, what will be the total number of moles of gasoline that had been diffused to the soil up to time t? (10 extra bonus points)