input: 
  def eval_expr(expr: str) -> float:
      """ Evaluates a given string containing a numerical expression and returns the result.
      >>> eval_expr("2+2")
      4.0
      >>> eval_expr("5*5")
      25.0
      >>> eval_expr("3.5/0.5")
      7.0
      """
  
output:     return float(eval(expr))