Derive the weakest precondition for the following assignment operator with a postcondition:

a $=3*($b $1)-1\{$ a $>5\}$

$10$

$($a$)$ Give a definition of ambiguous grammar.

$($b$)$ Explain why the following grammar is ambiguous:

$->$

$->+|$

$->$ a $|$b $|$ c

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