The function $f(x)$ satisfies
\[f(x + f(x)) = 4f(x)\]for all $x,$ and $f(1) = 4.$  Find $f(21).$
Question:
The function $f(x)$ satisfies
\[f(x + f(x)) = 4f(x)\]for all $x,$ and $f(1) = 4.$  Find $f(21).$
Answer:
Let's think step by step.

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The function $f(x)$ satisfies
\[f(x + f(x)) = 4f(x)\]for all $x,$ and $f(1) = 4.$  Find $f(21).$
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