adds chalet challenge

learning_python3.md

@@ -2100,7 +2100,46 @@ Take the additional class you made, such as vehicle and do some inheritance.
 For vehicles you could create a base class with a brand and number of wheels and seats.
 Then inherit for a bus, bike, car, cycle, etc.
 
-## A *practical* example
+# Coding challenge - Chalet floor
+
+Can you calculate me how much wood I need to order to build the base of my chalet?
+I have three different plans, you can see below.
+The idea behind this exercise is to create a `class` for each *base* shape.
+If you implement the *right* methods you can add, subtract, multiply your shapes as you wish.
+I created a library and imported it in my python shell so I can do the following.
+
+```python
+>>> import shapes
+>>> sq1 = shapes.
+shapes.Circle(     shapes.Rectangle(  shapes.Shape(      shapes.Square(     shapes.Triangle(   shapes.math
+>>> sq1 = shapes.Square(400)
+>>> cr1 = shapes.Circle(30)
+>>> re1 = shapes.Rectangle(400, 600)
+>>> total = sq1 + re1 - cr1
+>>> print(total)
+I'm a Shape with an area of 397172.57 cm2
+>>> print(re1)
+I'm a Rectangle with an area of 240000.00 cm2
+>>> print(cr1)
+I'm a Circle with an area of 2827.43 cm2
+>>> print(sq1)
+I'm a Square with an area of 160000.00 cm2
+>>> 
+```
+
+There are quite a few things in this exercise we have not seen in detail, so don't hesitate to research online and ask questions!
+This is a quite **advanced** challenge so you can be very proud of yourself if you understand the solution!
+
+![first plan](./assets/plan_01_topview.png)
+
+![second plan](./assets/plan_02_topview.png)
+
+![third plan](./assets/plan_03_topview.png)
+
+![third plan 3D view](./assets/plan_03_3d.png)
+
+<details>
+  <summary>Spoiler warning</summary>
 
 ```python
 import math
@@ -2167,6 +2206,8 @@ if __name__ == "__main__":
     print(combi)
 ```
 
+</details>
+
 ## Now some *practical* improvements
 
 ### Improve the login generator