Find all possible three digit numbers whose digits' sum equals 12

Question

How many 3 digit numbers exist such that the sum of their digits equals 12? I ran a little program and found that there are $66$ of such numbers.

I feel that this type of problem is similar in style to that of the Diophantine equation $ax+by=c$ except in this case $a+b+c=12$ where $a,b,c,$ are positive integers.
The only problem is that I don't know how properly solve a Diophantine equation or for that matter even solve one without brute force and tears.

I thought about making a table and finding a pattern between the number of possible orders by increasing the hundreds digit by 1 and then counting.

Answer 1

We can solve this with the stars and bars method.

We need to find all non-negative solutions $a+b+c=11$ with $a\leq 8$ and $b,c\leq 9$. (the number is going to be $\overline{(a+1)bc}$).

To do this we first find all the solutions disregarding the upper bound on a,b,c.

This is simple with stars and bars. There are $11$ stars and $2$ bars, so $\binom{13}{2}=78$.

We now need to subtract the solutions in which $a\geq 9,b\geq 10$ and $c\geq 10$.(but none of them happen at the same time, so it is easy)

There are $6$ when $a\geq9$, $3$ when $b\geq 10$ and $3$ when $c\geq 10$.

Therefore the final answer is $78-6-3-3=66$

Answer 2

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With the numbers $\ds{100,101,102,\ldots,997,998,999}$:

\begin{align} &\color{#f00}{\sum_{a = \color{#000}{\large 1}}^{9}\,\sum_{b = 0}^{9} \sum_{c = 0}^{9}\delta_{a + b + c,12}}\,\,\, = \sum_{a = 1}^{9}\sum_{b = 0}^{9}\sum_{c = 0}^{9}\,\,\oint_{\verts{z}\ =\ 1^{-}} \,\,\,{1 \over z^{13 - a - b - c}}\,\,\,{\dd z \over 2\pi\ic} \\[5mm] = &\ \oint_{\verts{z}\ =\ 1^{-}}\,\, {1 \over z^{13}}\pars{\sum_{a = 1}^{9}z^{a}} \pars{\sum_{\ell = 0}^{9}z^{\ell}}^{2}\,{\dd z \over 2\pi\ic} \\[5mm] = &\ \oint_{\verts{z}\ =\ 1^{-}}\,\,{1 \over z^{13}}\, \pars{z\,{z^{9} - 1 \over z - 1}}\,\pars{z^{10} - 1 \over z - 1}^{2} \,{\dd z \over 2\pi\ic} \\[5mm] = &\ \oint_{\verts{z}\ =\ 1^{-}}\,\, {\pars{z^{9} - 1}\pars{z^{10} - 1}^{2} \over z^{12}}\,{1 \over \pars{z - 1}^{3}} \,{\dd z \over 2\pi\ic} \\[5mm] = &\ \oint_{\verts{z}\ =\ 1^{-}}\,\, {z^{29} - z^{20} -2z^{19} + \color{#f00}{2z^{10}} + \color{#f00}{z^{9}} - \color{#f00}{1} \over z^{12}} \sum_{k = 0}^{\infty}{-3 \choose k}\pars{-1}^{k + 1}\,\,z^{k} \,{\dd z \over 2\pi\ic} \\[5mm] = &\ 2{-3 \choose 1} - {-3 \choose 2} - {-3 \choose 11} = -2{3 \choose 1} - {4 \choose 2} + {13 \choose 11} = -2 \times 3 - 6 + 78 = \color{#f00}{66} \end{align}