A function $f$ has the property that, for all $x$, $2f(x)=f(2x)$. If $f(10)=12$, what is the value of $f(5)$?

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We obtain a relationship between $f(10)$ and $f(5)$ when $x=5$.

$$2f(x)=f(2x)$$ $$2f(5)=f(2\cdot5)$$ $$2f(5)=f(10)$$ $$2f(5)=12$$ $$f(5)=\boxed{6}$$