9/10 = 0.9
83 & 9/10 = 83 + 9/10 = 83 + 0.9 = 83.9
(83 & 9/10)% = 83.9%
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Alex sold 83.9% of the lemonade, so 100% - 83.9% = 16.1% is left over
16.1% = 0.161 after moving the decimal point 2 spots to the left
the decimal value converts to the fraction 161/1000, which represents the fraction of the lemonade left over.
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Answer: 161/1000
