I needed help with number 20

20. Compute the weakest precondition for each of the following sequences of assignment statements and their postconditions: a. a -2*b+ 1; b=a-3 b, a = 3 * (2 * b + a); 21. Write a denotational semantics mapping function for the following statements: a. Ada for b. Java do-while c. Java Boolean expressions Java for e. C switch 22. What is the difference between an intrinsic attribute and a nonintrinsic synthesized attribute? 23. Write an attribute grammar whose BNF basis is that of Example 3.6 in Section 3.4.5, but whose language rules are as follows: Data types cannot be mixed in expressions, but assignment statements need not have the same types on both sides of the assignment operator Write an attribute grammar whose base BNF is that of Example 3.2 and whose type rules are the same as for the assignment statement example of Section 3.4.5 24