Có \(p+n+e=115\)
\(\Leftrightarrow2p+n=115\) (1)
Lại có:\(\dfrac{n}{p}=\dfrac{9}{7}\Rightarrow7n-9p=0\) (2)
Từ (1) (2) \(\Rightarrow\left\{{}\begin{matrix}n+2p=115\\7n-9p=0\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}n=45\\p=35\end{matrix}\right.\)
R là Brom