A certain chain store sells chocolate bars in packets of 17 and 9 only. Clearly, you could not get 8 or 25 bars. Find all quantities of bars that you cannot buy.

• this is clearly an ad for chocolate. I'll be back. – Timmerz Nov 8 '14 at 12:55

We can make $17 \times 8 = 136$ and any number above this by replacing a $17$ with $2 \times 9$, to increment the number by $1$. By the time we run out of $17$s to replace ($9 \times 16$), we continue with $17 \times 8 + 9 = 145$ and repeat the trick, until we arrive at $9 \times 17$, at which point we swap that with $17 \times 9$ and start replacing $17$s again.

Looking at the numbers below $136$, we can make any number $n \times 17$ and

1. the following $n$ integers by replacement of $17$ with $2 \times 9$,
2. anything $m\times9$ above this e.g.
$17$ gives us $18$ by replacement and $26$, $35$, $44$, $53$, $62$, $71$, $80$, $89$, $98$, $107$, $116$, $125$, $134$ by addition of $9$s

Adding all of these we find that the highest number we cannot make is

$127$

and there are

$63$

possible numbers below this.

The ones we cannot form are:

1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 19, 20, 21, 22, 23, 24, 25, 28, 29, 30, 31, 32, 33, 37, 38, 39, 40, 41, 42, 46, 47, 48, 49, 50, 55, 56, 57, 58, 59, 64, 65, 66, 67, 73, 74, 75, 76, 82, 83, 84, 91, 92, 93, 100, 101, 109, 110, 118, 127

• so what quantities can you not buy :p – d'alar'cop Nov 8 '14 at 10:35
• I know the question is not answered - I had a PEBKAC error and posted before I had finished writing. Finishing it now. – frodoskywalker Nov 8 '14 at 10:37
• +1 anyway of course. It's essentially solved. nice job :) – d'alar'cop Nov 8 '14 at 10:39
• I can't find a simple argument why these numbers below 128 cannot be gotten and all others can. Or do I miss this? – miracle173 Nov 8 '14 at 12:26
• @miracle You can get 1) Any multiple of 9, 2) Any multiple of 17 (call it $n*17$) and 3) the next $n$ numbers after (2). Once (2) and (3) start to overlap, you can form all higher numbers. – frodoskywalker Nov 8 '14 at 12:54

The numbers we can make are of the form $m \times 9 + n \times 17 \,| \, m, n \in \mathbb{Z}_{\ge0}$.

As we can see (and as has been said before), the largest number we can't make is $127$.

$$\begin{array}{r|rr} \text{target} & m & n \\ \hline 0 & 0 & 0 \\ 1 & - & - \\ 2 & - & - \\ 3 & - & - \\ 4 & - & - \\ 5 & - & - \\ 6 & - & - \\ 7 & - & - \\ 8 & - & - \\ 9 & 1 & 0 \\ 10 & - & - \\ 11 & - & - \\ 12 & - & - \\ 13 & - & - \\ 14 & - & - \\ 15 & - & - \\ 16 & - & - \\ 17 & 0 & 1 \\ 18 & 2 & 0 \\ 19 & - & - \\ 20 & - & - \\ 21 & - & - \\ 22 & - & - \\ 23 & - & - \\ 24 & - & - \\ 25 & - & - \\ 26 & 1 & 1 \\ 27 & 3 & 0 \\ 28 & - & - \\ 29 & - & - \\ 30 & - & - \\ 31 & - & - \\ 32 & - & - \\ 33 & - & - \\ 34 & 0 & 2 \\ 35 & 2 & 1 \\ 36 & 4 & 0 \\ 37 & - & - \\ 38 & - & - \\ 39 & - & - \\ 40 & - & - \\ 41 & - & - \\ 42 & - & - \\ 43 & 1 & 2 \\ 44 & 3 & 1 \\ 45 & 5 & 0 \\ 46 & - & - \\ 47 & - & - \\ 48 & - & - \\ 49 & - & - \\ 50 & - & - \\ 51 & 0 & 3 \\ 52 & 2 & 2 \\ 53 & 4 & 1 \\ 54 & 6 & 0 \\ 55 & - & - \\ 56 & - & - \\ 57 & - & - \\ 58 & - & - \\ 59 & - & - \\ 60 & 1 & 3 \\ 61 & 3 & 2 \\ 62 & 5 & 1 \\ 63 & 7 & 0 \\ 64 & - & - \\ 65 & - & - \\ 66 & - & - \\ 67 & - & - \\ 68 & 0 & 4 \\ 69 & 2 & 3 \\ 70 & 4 & 2 \\ 71 & 6 & 1 \\ 72 & 8 & 0 \\ 73 & - & - \\ 74 & - & - \\ 75 & - & - \\ 76 & - & - \\ 77 & 1 & 4 \\ 78 & 3 & 3 \\ 79 & 5 & 2 \\ 80 & 7 & 1 \\ 81 & 9 & 0 \\ 82 & - & - \\ 83 & - & - \\ 84 & - & - \\ 85 & 0 & 5 \\ 86 & 2 & 4 \\ 87 & 4 & 3 \\ 88 & 6 & 2 \\ 89 & 8 & 1 \\ 90 & 10 & 0 \\ 91 & - & - \\ 92 & - & - \\ 93 & - & - \\ 94 & 1 & 5 \\ 95 & 3 & 4 \\ 96 & 5 & 3 \\ 97 & 7 & 2 \\ 98 & 9 & 1 \\ 99 & 11 & 0 \\ 100 & - & - \\ 101 & - & - \\ 102 & 0 & 6 \\ 103 & 2 & 5 \\ 104 & 4 & 4 \\ 105 & 6 & 3 \\ 106 & 8 & 2 \\ 107 & 10 & 1 \\ 108 & 12 & 0 \\ 109 & - & - \\ 110 & - & - \\ 111 & 1 & 6 \\ 112 & 3 & 5 \\ 113 & 5 & 4 \\ 114 & 7 & 3 \\ 115 & 9 & 2 \\ 116 & 11 & 1 \\ 117 & 13 & 0 \\ 118 & - & - \\ 119 & 0 & 7 \\ 120 & 2 & 6 \\ 121 & 4 & 5 \\ 122 & 6 & 4 \\ 123 & 8 & 3 \\ 124 & 10 & 2 \\ 125 & 12 & 1 \\ 126 & 14 & 0 \\ 127 & - & - \\ \hline 128 & 1 & 7 \\ 129 & 3 & 6 \\ 130 & 5 & 5 \\ 131 & 7 & 4 \\ 132 & 9 & 3 \\ 133 & 11 & 2 \\ 134 & 13 & 1 \\ 135 & 15 & 0 \\ 136 & 0 & 8 \\ 137 & 2 & 7 \\ 138 & 4 & 6 \\ 139 & 6 & 5 \\ 140 & 8 & 4 \\ 141 & 10 & 3 \\ 142 & 12 & 2 \\ 143 & 14 & 1 \\ 144 & 16 & 0 \\ 145 & 1 & 8 \\ 146 & 3 & 7 \\ 147 & 5 & 6 \\ 148 & 7 & 5 \\ 149 & 9 & 4 \\ 150 & 11 & 3 \\ 151 & 13 & 2 \\ 152 & 15 & 1 \\ 153 & 17 & 0 \\ 154 & 2 & 8 \\ 155 & 4 & 7 \\ 156 & 6 & 6 \\ 157 & 8 & 5 \\ 158 & 10 & 4 \\ 159 & 12 & 3 \\ 160 & 14 & 2 \\ 161 & 16 & 1 \\ 162 & 18 & 0 \\ 163 & 3 & 8 \\ 164 & 5 & 7 \\ 165 & 7 & 6 \\ 166 & 9 & 5 \\ 167 & 11 & 4 \\ 168 & 13 & 3 \\ 169 & 15 & 2 \\ 170 & 17 & 1 \\ 171 & 19 & 0 \\ \end{array}$$