Simplify $\sqrt{420 - \sqrt{420 - \sqrt{420 - \sqrt{420 - \dotsb}}}}$.
\(x=\sqrt{420 - \sqrt{420 - \sqrt{420 - \sqrt{420 - \dotsb}}}}\\x^2=420-x\\x^2+x-420=0\\(x-20)(x+21)=0\\\boxed{x=20}\)
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