code: def progression_linear(week, start_weight, final_weight, start_week, end_week):
      
      # Calculate the slope of the linear function
      slope = (start_weight - final_weight) / (start_week - end_week)
  
      # Return the answer y = slope (x - x_0) + y_0
      return slope * (week - start_week) + start_weight
docs: A linear progression function going through the points
      ('start_week', 'start_weight') and ('end_week', 'final_weight'), evaluated
      in 'week'.
  
      Parameters
      ----------
      week
          The week to evaluate the linear function at.
      start_weight
          The weight at 'start_week'.
      final_weight
          The weight at 'end_week'.
      start_week
          The number of the first week, typically 1.
      end_week
          The number of the final week, e.g. 8.
  
  
      Returns
      -------
      weight
          The weight at 'week'.
  
  
      Examples
      -------
      >>> progression_linear(week = 2, start_weight = 100, final_weight = 120,
      ...                    start_week = 1, end_week = 3)
      110.0
      
      >>> progression_linear(3, 100, 140, 1, 5)
      120.0