Inferring relative item prices from rounded combined transaction totals (indeterminacy due to rounding).
Adding all three given totals: (file+notebook) + (notebook+pen) + (pen+file) = 27+31+29 = 87, so 2(file+notebook+pen)=87, giving file+notebook+pen=43.5. This implies pen=43.5-27=16.5, file=43.5-31=12.5, notebook=43.5-29=14.5 for the...
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