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MathNet 2018-part-07 — 177 problems (EN–VI) Question 143

Math Olympiad Vault · MathNet 2018-part-07 — 177 problems (EN–VI)

Question 143

I have two cents and Bill has $n$ cents. Bill wants to buy some pencils, which come in two different packages. One package of pencils costs 6 cents for 7 pencils, and the other package of pencils costs a dime for a dozen pencils (i.e. 10 cents for 12 pencils). Bill notes that he can spend all $n$ of his cents on some combination of pencil packages to get $P$ pencils. However, if I give my two cents to Bill, he then notes that he can instead spend all $n+2$ of his cents on some combination of pencil packages to get fewer than $P$ pencils. What is the smallest value of $n$ for which this is possible?

Note: Both times Bill must spend all of his cents on pencil packages, i.e. have zero cents after either purchase.