Let A and B be 2 problems such that A ≤m B, and B is decidable. Let f be a mapping
reduction from A to B, and let D be a decider for B. Show that A is decidable by
constructing a decider for A.
Note: This problem is asking you to provide a constructive proof of a theorem in the
textbook -- specifically, Theorem 5.22, which states that if A ≤m B and B is decidable,
then A is decidable. You will not get credit for citing this theorem in your solution to
this problem.