Chamando de [tex]m_a[/tex] a massa do alumínio e de [tex]m_o[/tex] a massa do oxigênio, temos:
[tex]\dfrac{m_a}{m_o}=\dfrac{7}{8}\Longrightarrow8m_a=7m_o\Longrightarrow m_o=\dfrac{8m_a}{7}\\\\\\
m_a+m_o=51\Longrightarrow m_a+\dfrac{8m_a}{7}=51\Longrightarrow 7m_a+8m_a=357\\\\
\Longrightarrow 15m_a=357\Longrightarrow m_a=\dfrac{357}{15}\Longrightarrow\boxed{m_a=23,8\;g}\\\\\\
m_a+m_o=51\Longrightarrow 23,8+m_o=51\Longrightarrow \boxed{m_o=27,2\;g}
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