9/10 = 0.9
83 & 9/10 = 83 + 9/10 = 83 + 0.9 = 83.9
(83 & 9/10)% = 83.9%
------------------------
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.
-----------------------
Answer: 161/1000