\(\left\{{}\begin{matrix}P+E+N=42\\P=E\\P\le N\le1,5P\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2P+N=42\\P\le N\le1,5P\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}N=42-2P\\3P\le42\le3,5P\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}N=42-2P\\12\le P\le14\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}P=E=Z=12;N=18\left(LOẠI\right)\\P=E=Z=13;N=16\left(LOẠI\right)\\P=E=Z=14;N=14\left(Nhận:^{28}_{14}Si\right)\end{matrix}\right.\)