input: Write a function to take a string containing a numerical expression and return its result. 
  
  def eval_expr(expr):
     return eval(expr)
  
  assert eval_expr("3 + 5 * 2") == 13
  assert eval_expr("10 - 2 / 2") == 9
  assert eval_expr("8 * (2 + 3)") == 40
  assert eval_expr("20 / 4 - 3") == 2
  assert eval_expr("5 ** 2") == 25
output: def eval_expr(expr):
     return eval(expr)