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Combinatorics Difficulty 5.0 AIME Find the answer

Example 3. How many non-negative integer solutions does the equation x+3y+4z=665x+3 y+4 z=665 have?

Translate the above text into English, please keep the original text's line breaks and format, and output the translation result directly.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let x=12x1+r1(0r111)x=12 x_{1}+r_{1}\left(0 \leqslant r_{1} \leqslant 11\right),
y=4y1+r2(0r23),z=3z1+r3(0r32). \begin{array}{l} y=4 y_{1}+r_{2}\left(0 \leqslant r_{2} \leqslant 3\right), \\ z=3 z_{1}+r_{3}\left(0 \leqslant r_{3} \leqslant 2\right) . \end{array}

The equation becomes
12(x1+y1+z1)+r1+3r2+4r3=665.r1+3r2+4r36655(mod12).0r1+3r2+4r328,r1+3r2+4r3=5 or r1+3r2+4r3=17. \begin{array}{ll} & 12\left(x_{1}+y_{1}+z_{1}\right)+r_{1}+3 r_{2}+4 r_{3}=665 . \\ & \therefore r_{1}+3 r_{2}+4 r_{3} \equiv 665 \equiv 5(\bmod 12) . \\ & \because 0 \leqslant r_{1}+3 r_{2}+4 r_{3} \leqslant 28, \\ & r_{1}+3 r_{2}+4 r_{3}=5 \\ \text { or } & r_{1}+3 r_{2}+4 r_{3}=17 . \end{array}

It is easy to find that (1) has 3 solutions, (2) has 8 solutions.
\therefore The original equation has 3C655 52212+8C665521+2123 C_{\frac{655 ~ 5}{2}-2}^{12}+8 C_{\frac{665-5}{2}-1+2}^{12} =17108=17108 non-negative integer solutions.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.