In that recapitulative document, we sum up the following elements:

1. How to rewrite a simple puzzle as a system of two linear equations in two real variables.
2. How to solve the system obtained with a method of elimination and substitution.
3. How to transform the linear system into a matrix equation.
4. How to solve the matrix equation obtained directly with the inversion of a matrix.
5. How to set and solve the matrix equation in Python, with the packages ‘numpy’ and ‘numpy.linalg’.