To make the charges for the cab company a non-proportional situation, they would need to introduce an element that means the total cost does not vary directly with distance traveled.
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Charge a flat rate of $20 and then $12 per mile: This option introduces a flat fee of $20 that is charged regardless of the distance traveled. This means that even if a customer travels 0 miles, they still pay $20. The remaining cost per mile ($12/mile) is proportional, but the addition of the flat fee means that the overall charge no longer has a constant ratio with distance. Therefore, this option would create a non-proportional relationship.
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Charge $4 per mile instead of $12: This option simply changes the cost per mile to a lower rate but does not introduce any flat fee or other fixed cost. The relationship here remains proportional, as the charge will still vary directly with the distance traveled.
Thus, to make the charges non-proportional, the correct change would be to charge a flat rate of $20 and then $12 per mile.