DP Math AI · HL / SL · Number and Algebra

SL 1.7—Loan repayments and amortization

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  1. Question 1

    A student takes out a loan of $\$12{,}000$ to purchase a motorcycle. The loan has a nominal annual interest rate of 9%, compounded monthly, and is to be repaid over 3 years with equal monthly payments. What is the monthly payment amount, correct to the nearest cent?
    No clue? Show me the answer
    Correct answerCorrect!Incorrect
    A$381.60

    Step-by-step walkthrough

    Choose a solution method

    Method #1Approach 1

    Step 1: Identify the Variables

    Principal: P=12,000. Annual interest rate: 9%, so monthly rate: r=120.09​=0.0075. Number of payments: n=3×12=36.

    Step 2: Apply the Loan Payment Formula

    Payment=P×(1+r)n−1r(1+r)n​=12,000×(1.0075)36−10.0075×(1.0075)36​

    Step 3: Calculate the Compounding Factor

    (1.0075)36≈1.30865. This is the key exponential term accounting for compound interest over 36 months.

    Step 4: Complete the Calculation

    Payment=12,000×1.30865−10.0075×1.30865​=12,000×0.308650.009815​≈12,000×0.031800≈381.60

    Step 5: State the Answer

    The monthly payment is \381.60$. Using a GDC TVM Solver with N = 36, I% = 9, PV = 12000, FV = 0, P/Y = 12, C/Y = 12 and solving for PMT confirms this result.

    Method #2Approach 2

    Step 1: Identify What Is Being Asked

    We need the equal monthly payment for a \12{,}000loanat9%$ annual interest compounded monthly over 3 years. The correct approach uses the compound interest loan payment formula, not simple interest.

    Step 2: Eliminate $\$333.33$

    **\333.33∗∗comesfromsimplydividing$12{,}000$ by 36 payments. This ignores interest entirely and is incorrect for any interest-bearing loan.

    Step 3: Eliminate $\$360.00$

    **\360.00∗∗mightcomefromadding9%of$12{,}000($1{,}080$) and dividing by 36, which uses simple interest logic. Real loans use compound interest, so this underestimates the true payment.

    Step 4: Eliminate $\$392.44$

    **\392.44∗∗resultsfromincorrectlyusingthefullannualrateof0.09(insteadofthemonthlyrate0.0075$) in the formula, which significantly overstates the periodic rate and inflates the payment.

    Step 5: Select the Correct Answer

    **\381.60∗∗istheresultofcorrectlyapplyingtheloanpaymentformulawithr = 0.0075andn = 36$, giving the only accurate monthly payment.

  2. Question 2

    The table below shows the first two rows of an amortization schedule for a loan repaid monthly. Based on this information, what is the remaining balance after payment 2?

    Payment #PaymentInterest PaidPrincipal PaidRemaining Balance
    1\310.00$\100.00$\210.00$\19{,}790.00$
    2\310.00$???
    No clue? Show me the answer
    Correct answerCorrect!Incorrect
    B$19,578.95

    Step-by-step walkthrough

    Choose a solution method

    Method #1Approach 1

    Step 1: Identify the Monthly Interest Rate

    From payment 1, the original balance must have been \20{,}000(sinceprincipalpaid= $210.00andremainingbalance= $19{,}790.00,sostartingbalance= $20{,}000).Theinterestrateperperiodisr = \dfrac{100}{20{,}000} = 0.005$ per month.

    Step 2: Calculate Interest for Payment 2

    Interest in payment 2 = 19{,}790.00 \times 0.005 = \98.95$.

    Step 3: Calculate Principal Paid in Payment 2

    Principal paid in payment 2 = 310.00 - 98.95 = \211.05$.

    Step 4: Calculate Remaining Balance After Payment 2

    New balance = 19{,}790.00 - 211.05 = \19{,}578.95$.

    Step 5: State the Answer

    The remaining balance after payment 2 is \19{,}578.95$. Notice the balance decreases slightly more in period 2 than period 1 because less interest was charged.

    Method #2Approach 2

    Step 1: Identify the Key Calculation Required

    We must find the balance after payment 2 by computing: Interest = balance × rate, then Principal = payment − interest, then New balance = old balance − principal paid. The monthly rate is r=100/20,000=0.005.

    Step 2: Eliminate $\$19{,}579.05$

    **\19{,}579.05∗∗resultsfromasmallarithmeticroundingerrorincomputing19{,}790 \times 0.005 = 98.95as98.90.Thecorrectinterestis$98.95,makingthisansweroffby$0.10$.

    Step 3: Eliminate $\$19{,}580.00$

    **\19{,}580.00∗∗wouldresultfromsubtracting$210.00$ (the period-1 principal) again in period 2, ignoring that the interest changes each period. The principal paid increases slightly each period.

    Step 4: Eliminate $\$19{,}481.00$

    **\19{,}481.00∗∗resultsfromincorrectlysubtractingthefullpaymentof$310.00$ from the balance instead of only the principal portion, which confuses the total payment with the principal reduction.

    Step 5: Select the Correct Answer

    **\19{,}578.95∗∗iscorrect:interest= $98.95,principal= $211.05,newbalance= 19{,}790.00 - 211.05 = $19{,}578.95$.

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