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Algebra Level 1

$\begin{array} { c c c c c c c c c c } a & + & b & & & & & = & 3 \\ & & b & + & c & & & = & 7 \\ & & & & c & + & d & = & 9 \\ a & & & + & & & d & = & ? \\ \end{array}$

5 7 11 13

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by Chung Kevin

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