Prof. Forget used to make tricky questions for students. He gave postorder traversal sequence x y z $+$ w $*+$ for a tree of an expression. He also gave a formation of this tree with labels $\{1,2,3,4,5,6,7\}$ as follows.

$($a$)$ If x $=10,$ y $=20,$ z $=30,$ and w $=40,$ then identify the value after evaluating this expression.

$($b$)$ Write values of labels $1=\_\_,...,7=\_\_.(5$ Marks$)$