Let P(n) be the statement that a postage of n cents can be formed using just 4-cent stamps and 7-cent stamps. The Induction and Recursion parts of this exercise outline a strong induction proof that P(n) is true for n > 18. (a) Show that P(18), P(19), and P(20) are true, which com- pletes the base case.
(b) What is the inductive hypothesis?
(c) What do you need to prove in the inductive step?
(d) Complete the inductive step for k > 20.

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