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A prepaid meter starts with 240 units of credit and loses 12 units each day, so the credit left follows the rule U = 240 - 12t. What does the minus sign in front of the 12 tell you?
AThe credit stays level as the days pass
BThe credit rises quickly then levels off
CThe credit falls only after 240 days pass
DThe credit falls as the number of days rises
Answer & Solution
Correct answer: D. The credit falls as the number of days rises
1. The number in front of the input is the rate of change of the rule.
2. A negative rate of change means the output drops each time the input goes up by one.
3. Here the input is days and the output is credit, so credit falls by 12 units per day from the first day onwards.
4. Option C invents a delay that the rule does not contain, since the loss starts straight away.
5. So the credit falls as the days rise, option D.
_Source: OpenStax College Algebra (CC BY 4.0), Ch 4 "Linear Functions", section 4.1 Linear Functions_
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