Question
Find the rational number whose decimal expansion is $0.\overline{423}.$

Answer

$0.\overline{4.23}=0.4+0.0232323\ \dots$ $=0.4+0.023+0.00023+\ \dots$ $=0.4+\frac{23}{10^3}+\frac{23}{10^5}+\dots$ $=0.4+\frac{23}{10^3}\Big(1+\frac{1}{10^2}+\frac{1}{10^4}+\dots\Big)$ $=0.4+\frac{23}{1000}\Bigg(\frac{1}{1-\frac{1}{100}}\Bigg)$ $=0.4+\frac{23}{1000}\Big(\frac{100}{99}\Big)$ $=\frac{4}{10}+\frac{23}{990}$ $=\frac{360+23}{990}$ $0.\overline{423}=\frac{419}{990}$

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