Heat through a wall of two layers

A wall of $15\ \text{m}^2$ is $20\ \text{cm}$ of brick. How much mineral wool should line it? You will make the program find the heat flow for wool from $0\ \text{cm}$ to $15\ \text{cm}$ thick, with $20\ ^\circ\text{C}$ inside and $2\ ^\circ\text{C}$ outside, and the thickness that cuts the loss to a quarter.

The idea. Each layer has a thermal resistance $R = \dfrac{L}{kA}$; layers in series add. The heat flow is $H = \dfrac{\Delta T}{R_{\text{total}}}$. Brick $k = 0.60\ \text{W m}^{-1}\text{K}^{-1}$, wool $0.04\ \text{W m}^{-1}\text{K}^{-1}$.

Your tasks.

1. Work out the brick's resistance R_brick and the wool's resistance R_wool for every thickness in x.
2. Work out the heat flow H for every thickness.
3. Find the thinnest wool that cuts the heat flow to a quarter of the bare brick's.