Mickey's error lies in the way he calculated the unit price and how he treated the division in his initial steps. Let's break it down:
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Misinterpretation of Division: Mickey used the division operation incorrectly. When he wrote "12 cartons(÷22.20)" and then "$22.20(÷22.20)", he did not arrive at the correct unit price of the eggs. Instead, he should have divided the total cost by the number of cartons to find the price per carton.
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Correct Calculation of Unit Price:
- The unit price should be calculated as: \[ \text{Unit Price} = \frac{\text{Total Cost}}{\text{Number of Cartons}} = \frac{22.20}{12} \]
- This calculation results in: \[ \text{Unit Price} = 1.85 , (\text{approximately}) \]
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Calculating the Total Cost for 7 Cartons:
- After finding the correct unit price of approximately $1.85 per carton, he would then multiply this unit price by 7 to find out how much he would spend: \[ \text{Total Cost for 7 Cartons} = 1.85 \times 7 \approx 12.95 \]
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Mickey's Incorrect Total: Mickey incorrectly arrived at a unit price of $0.54 and then calculated the cost for 7 cartons as $3.78. These results are wrong due to the miscalculation of the unit price.
In summary, Mickey's error was in the method he used to calculate the unit price, leading to a chain of incorrect subsequent calculations.