Ta có: \(p+e+n=18\)
\(\Leftrightarrow2p+n=18\left(I\right)\)
M\(\ne\): \(\dfrac{Z}{3,5}\le p\le\dfrac{Z}{3}\)
\(\Leftrightarrow\dfrac{18}{3,5}\le p\le\dfrac{18}{3}\)
\(\Leftrightarrow5,1\le p\le6\)
\(\Rightarrow p=e=6\)
Thay vào (I) \(\Rightarrow n=6\)