Larry has 4-cent stamps and 9-cent stamps, which he can combine to produce various amounts of postage. For example, he can make cents by using four 9-cent stamps and a 4-cent stamp, or by using ten 4-cent stamps. However, there are some amounts of postage he can't make exactly, such as 10 cents.
What is the largest number of cents that Larry cannot make exactly from a combination of 4- and/or 9-cent stamps?
Explain how you know your answer is correct. (You should explain two things: why Larry can't make the amount of your answer, and why he can make any bigger amount.)
See the answer here: https://web2.0calc.com/questions/math-postage-stamp-problem