3. Strong Induction (10 points) (1) (6 points) Let P(n) be the statement that a postage of n cents can be formed using just 4-cent stamps and 7-cnet stamps. The parts of this exercise outline a strong induction proof that P(n) is true for n 2 18. (a) Show statements P(18), P(19), P(20), and P(21) are true, complet- ing the basis step of the proof. (b) What is the inductive hypothesis of the proof? (c) What do you need to prove in the inductive step? (d) Complete the inductive step for k 221