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Marlene has a credit card that uses the adjusted balance method. For the first 10 days of one of her 30-day billing cycles, her balance was [tex]$\$[/tex]570[tex]$. She then made a purchase for $[/tex]\[tex]$120$[/tex], so her balance jumped to [tex]$\$[/tex]690[tex]$, and it remained that amount for the next 10 days. Marlene then made a payment of $[/tex]\[tex]$250$[/tex], so her balance for the last 10 days of the billing cycle was [tex]$\$[/tex]440[tex]$. If her credit card's APR is $[/tex]15\%[tex]$, which of these expressions could be used to calculate the amount Marlene was charged in interest for the billing cycle?

A. $[/tex]\left(\frac{0.15}{365} \cdot 30\right)(\[tex]$320)$[/tex]

B. [tex]$\left(\frac{0.15}{365} \cdot 30\right)(\$[/tex]570)[tex]$

C. $[/tex]\left(\frac{0.15}{365} \cdot 30\right)\left(\frac{10 \cdot \[tex]$570 + 10 \cdot \$[/tex]690 + 10 \cdot \[tex]$250}{30}\right)$[/tex]

D. [tex]$\left(\frac{0.15}{365} \cdot 30\right)\left(\frac{10 \cdot \$[/tex]570 + 10 \cdot \[tex]$690 + 10 \cdot \$[/tex]440}{30}\right)$


Sagot :

To determine the amount Marlene was charged in interest for the billing cycle, we need to follow these steps:

1. Identify the relevant balances and the number of days each balance was held:
- For the first 10 days, her balance was [tex]\( \$570 \)[/tex].
- For the next 10 days, her balance was [tex]\( \$690 \)[/tex] after making a \[tex]$120 purchase. - For the final 10 days, her balance was \( \$[/tex]440 \) after making a \[tex]$250 payment. 2. Calculate the average daily balance: The average daily balance is found by averaging the balances weighted by the number of days each balance was held. \[ \text{Average Daily Balance} = \frac{(10 \text{ days} \times \$[/tex]570) + (10 \text{ days} \times \[tex]$690) + (10 \text{ days} \times \$[/tex]440)}{10 + 10 + 10}
\]
[tex]\[ = \frac{10 \times 570 + 10 \times 690 + 10 \times 440}{30} \][/tex]
[tex]\[ = \frac{5700 + 6900 + 4400}{30} \][/tex]
[tex]\[ = \frac{17000}{30} \][/tex]
[tex]\[ = 566.6666666666666 \][/tex]

3. Determine the daily interest rate:
The Annual Percentage Rate (APR) is given as [tex]\( 15\% \)[/tex] (0.15 as a decimal), and this must be converted to a monthly interest rate since the billing cycle is for a month:
[tex]\[ \text{Daily Interest Rate} = \frac{APR}{365} \times 30 \][/tex]
[tex]\[ = \frac{0.15}{365} \times 30 \][/tex]
[tex]\[ = 0.012328767123287671 \][/tex]

4. Calculate the interest charged:
Finally, multiply the average daily balance by the daily interest rate:
[tex]\[ \text{Interest Charged} = \text{Daily Interest Rate} \times \text{Average Daily Balance} \][/tex]
[tex]\[ = 0.012328767123287671 \times 566.6666666666666 \][/tex]
[tex]\[ = 6.986301369863013 \][/tex]

5. Verify the correct expression:
We need to determine which expression matches the correct steps we've taken for calculating the interest charged.

The correct expression from the given choices is:
[tex]\[ \left( \frac{0.15}{365} \times 30 \right) \left( \frac{10 \times \$570 + 10 \times \$690 + 10 \times \$440}{30} \right) \][/tex]

This corresponds to the expression in option D.

Therefore, the correct expression is [tex]\( \left( \frac{0.15}{365} \times 30 \right) \left( \frac{10 \times \$570 + 10 \times \$690 + 10 \times \$440}{30} \right) \)[/tex].