Gọi $x$, $y$, $z$ là số mol $Fe$, $Mg$, $Cu$
$\Rightarrow 56x+24y+64z=23,7$ $(1)$
$Fe$ không tan trong $H_2SO_4$ đặc nguội.
$n_{SO_2}=\dfrac{10,1}{22,4}=0,45(mol)$
Bảo toàn e: $2n_{Mg}+2n_{Cu}=2n_{SO_2}$
$\Rightarrow y+z=0,45$ $(2)$
$n_{H_2}=\dfrac{3,7}{22,4}=0,165(mol)$
Bảo toàn e: $2n_{Fe}=2n_{H_2}$
$\Rightarrow x=0,165$ $(3)$
$(1)(2)(3)\Rightarrow x=0,165; y=0,3585; z=0,0915$
$\%m_{Fe}=\dfrac{56x.100}{23,7}=39\%$
$\%m_{Mg}=\dfrac{24y.100}{23,7}=36,3\%$
$\%m_{Cu}=24,7\%$