Question 1173028
Let's calculate the interest charged by each card.

**Silver Card (18% interest, balance on the first day of the previous month):**

* **October:**
    * Balance on October 1st: $0
    * Purchase on October 23rd: $1,800
    * Interest for October: $0 (since the balance on Oct 1 was 0)
* **November:**
    * Balance on November 1st: $1,800
    * Interest for November: $1,800 * (0.18 / 12) = $27
    * Payment on November 10th: $600
    * Remaining balance: $1,800 + $27 - $600 = $1,227
* **December:**
    * Balance on December 1st: $1,227
    * Interest for December: $1,227 * (0.18 / 12) = $18.41
    * Payment on December 10th: $600
    * Remaining balance: $1,227 + $18.41 - $600 = $645.41
* **January:**
    * Balance on January 1st: $645.41
    * Interest for January: $645.41 * (0.18 / 12) = $9.68
    * Payment on January 10th: $645.41+$9.68 = $655.09
* **Total Interest (Silver Card):** $27 + $18.41 + $9.68 = $55.09

**Gold Card (18% interest, average daily balance):**

* **October:**
    * Days with $0 balance: 22
    * Days with $1,800 balance: 8 (including Oct 23rd)
    * Average daily balance: [(22 * $0) + (8 * $1,800)] / 30 = $1,440
    * Interest for October: $1,440 * (0.18 / 12) = $21.60
* **November:**
    * Balance at the start of Nov: $1,800 + $21.60 = $1,821.60
    * Days with 1821.60 balance: 9.
    * Balance after payment: $1,821.60 - $600 = $1,221.60.
    * Days with 1221.60 balance: 21
    * Average daily balance: [(9 * $1,821.60) + (21 * $1,221.60)] / 30 = $1,401.60
    * Interest for November: $1,401.60 * (0.18 / 12) = $21.02
* **December:**
    * Balance at the start of Dec: $1,221.60 + $21.02 = $1,242.62
    * Days with 1242.62 balance: 9.
    * Balance after payment: $1,242.62 - $600 = $642.62
    * Days with $642.62 balance: 21
    * Average daily balance: [(9 * $1,242.62) + (21 * $642.62)] / 30 = $822.62
    * Interest for December: $822.62 * (0.18 / 12) = $12.34
* **January:**
    * Balance at start of January: $642.62 + $12.34 = $654.96
    * Days with 654.96 balance: 10.
    * Average daily balance: $654.96
    * Interest for January: $654.96 * (0.18 / 12) = $9.82
* **Total Interest (Gold Card):** $21.60 + $21.02 + $12.34 + $9.82 = $64.78

**Answers:**

* Silver card: $55.09
* Gold card: $64.78