Question: Find the flaw with the following "proof" that every postage of three cents or more can be formed using just three-cent and four-cent stamps. Basis Step: We can form postage of three cents with a single three-cent stamp and we can form postage of four cents using a single four-cent stamp. Inductive Step:
Assume that we can form postage of j cents for all nonnegative integers j with j ≤ k using just three-cent and four-cent stamps. We can then form postage of k + 1 cents by replacing one three-cent stamp with a four-cent stamp or by replacing two fourcent stamps by three three-cent stamps.