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Terry has a credit card that uses the average daily balance method. For the first 18 days of one of his billing cycles, his balance was [tex]$\$[/tex] 350[tex]$, and for the last 12 days of the billing cycle, his balance was $[/tex]\[tex]$ 520$[/tex]. If his credit card's APR is [tex]$14\%$[/tex], which of these expressions could be used to calculate the amount Terry was charged in interest for the billing cycle?

A. [tex]$\left(\frac{0.14}{365} \cdot 31\right)\left(\frac{12 \cdot \$[/tex] 350+18 \cdot \[tex]$ 520}{31}\right)$[/tex]
B. [tex]$\left(\frac{0.14}{365} \cdot 30\right)\left(\frac{12 \cdot \$[/tex] 350+18 \cdot \[tex]$ 520}{30}\right)$[/tex]
C. [tex]$\left(\frac{0.14}{365} \cdot 30\right)\left(\frac{18 \cdot \$[/tex] 350+12 \cdot \[tex]$ 520}{30}\right)$[/tex]
D. [tex]$\left(\frac{0.14}{365} \cdot 31\right)\left(\frac{18 \cdot \$[/tex] 350+12 \cdot \[tex]$ 520}{31}\right)$[/tex]


Sagot :

Let's solve this step-by-step.

### Step 1: Calculate the Average Daily Balance

1. Days with a balance of \[tex]$350: 18 days 2. Days with a balance of \$[/tex]520: 12 days
3. Total days in the billing cycle: 30 days

The average daily balance (ADB) can be calculated as follows:

[tex]\[ \text{ADB} = \frac{(\text{Days with balance of } \$350)\times \$350 + (\text{Days with balance of } \$520)\times \$520}{\text{Total days in cycle}} \][/tex]

Plug in the values:

[tex]\[ \text{ADB} = \frac{(18 \times 350) + (12 \times 520)}{30} \][/tex]

Calculate inside the parentheses:

[tex]\[ \text{ADB} = \frac{(6300) + (6240)}{30} \][/tex]

Add the numbers:

[tex]\[ \text{ADB} = \frac{12540}{30} = 418 \][/tex]

So, the average daily balance is \[tex]$418. ### Step 2: Calculate the Daily Interest Rate The annual percentage rate (APR) is 14%, which we convert to a decimal: \[ \text{APR} = 0.14 \] Since interest is charged daily, we need to find the daily interest rate by dividing the APR by the number of days in a year (365): \[ \text{Daily Interest Rate} = \frac{0.14}{365} \approx 0.000383561643835616 \] ### Step 3: Calculate the Interest Charged Now we use the daily interest rate to calculate the interest charged over the billing cycle. The billing cycle has 30 days: \[ \text{Interest Charged} = \text{Daily Interest Rate} \times \text{Number of Days in Cycle} \times \text{Average Daily Balance} \] Substitute the values: \[ \text{Interest Charged} = 0.000383561643835616 \times 30 \times 418 \approx 4.809863013698631 \] So, the interest charged is approximately \$[/tex]4.81.

### Step 4: Verify the Given Expressions

We now need to verify which of the provided expressions match our calculation:

#### Option A:
[tex]\[ \left(\frac{0.14}{365} \cdot 31\right)\left(\frac{12 \times 350 + 18 \times 520}{31}\right) \][/tex]

Evaluate the inside expression:

[tex]\[ \frac{12 \times 350 + 18 \times 520}{31} = \frac{(4200 + 9360)}{31} = \frac{13560}{31} \approx 437.741935 \][/tex]

And the overall expression:

[tex]\[ (0.000383561643835616 \times 31) \times 437.741935 \approx 5.20109589041096 \][/tex]

#### Option B:
[tex]\[ \left(\frac{0.14}{365} \cdot 30\right)\left(\frac{12 \times 350 + 18 \times 520}{30}\right) \][/tex]

Evaluate the inside expression:

[tex]\[ \frac{12 \times 350 + 18 \times 520}{30} = 418 \][/tex]

And the overall expression:

[tex]\[ (0.000383561643835616 \times 30) \times 418 \approx 4.809863013698631 \][/tex]

#### Option C:
[tex]\[ \left(\frac{0.14}{365} \cdot 30\right)\left(\frac{18 \times 350 + 12 \times 520}{30}\right) \][/tex]

Evaluate the inside expression:

[tex]\[ \frac{18 \times 350 + 12 \times 520}{30} = 418 \][/tex]

And the overall expression:

[tex]\[ (0.000383561643835616 \times 30) \times 418 \approx 4.809863013698631 \][/tex]

#### Option D:
[tex]\[ \left(\frac{0.14}{365} \cdot 31\right)\left(\frac{18 \times 350 + 12 \times 520}{31}\right) \][/tex]

Evaluate the inside expression:

[tex]\[ \frac{18 \times 350 + 12 \times 520}{31} = \frac{12540}{31} \approx 404.193548387 \][/tex]

And the overall expression:

[tex]\[ (0.000383561643835616 \times 31) \times 404.193548387 \approx 4.809863013698631 \][/tex]

### Conclusion

The expressions that could be used to calculate the amount charged in interest are:

[tex]\[ \boxed{\text{B},\, \text{C},\, \text{D}} \][/tex]