Sherri uses her credit card to pay for new transmission that ended up totaling $1,050.25. she can pay off up to $400 a month. the card has an annual rate of 18.5%. how much will she pay in interest?

Nice practical question.
Please tell Sherri that it is less expensive to get a bank loan than to use the credit card.

Principal, P=1050.25
interest rate, i = 18.5%/12 per month
Monthly Repayment, A = 400

We need to calculate the monthly balance to know how many months it takes her to pay off the debit, and hence the interest charge.

As usual, we assume the debit is made at the beginning of the month, and the $400 paid at the end of the month.  In actual practice, all these debits and credits are calculated on a daily balance basis, and interest accumulated at the end of the month.

The monthly interest is calculated as the balance from last month multiplied by the interest rate, 18.5%/12.  The amount owing (at the end of the month)
F=Balance(1+0.185/12)

month    Owing    Paid    Balance    Calculation of owing next month
0           1050.25     0      1050.25     1050.25*(1+0.185/12)=1066.44
1            1066.44  400      666.44     666.44(1+0.185/12)=676.72
2              676.72  400      276.72     276.72(1+0.185/12)=280.98
3              280.98  290.98    loan paid off

Total interest paid = 400+400+290.98 - 1050.25 = 40.73