$$\frac{ 12x }{ 100 } + \frac{ 10 \times (56000-x) }{ 100 } =6400$$
$x=40000$
$$12x+10\left(56000-x\right)=640000$$
$$12x+560000-10x=640000$$
$$2x+560000=640000$$
$$2x=640000-560000$$
$$2x=80000$$
$$x=\frac{80000}{2}$$
$$x=40000$$
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