p+n+e=40
=>2p+n=40
=> n=40-2p
Mặt khác
p\(\le n\le1,5p\)
=> p\(\le40-2p\le1,5p\)
=>3p\(\le40\le3,5p\)
=> 11,43\(\le p\le13,33\)
=> p=11,12,13
| 11 | Na |
| 12 | Mg |
| 13 | Al |
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