Any collection of homothetic copies of some tetrahedron Tr\documentclass[12pt]{minimal}
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\begin{document}$$T_r$$\end{document}, whose total volume does not exceed one quarter of the volume of Tr\documentclass[12pt]{minimal}
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\begin{document}$$T_r$$\end{document}, can be packed into Tr\documentclass[12pt]{minimal}
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\begin{document}$$T_r$$\end{document} .