Jeremy plans to invest $100 000 at the beginning of year 2024 into a current account which pays compound interest at a rate of 3.2% per annum. The interest is paid on the last day of each year based on the amount in the account. He also puts a further $X into the current account on the first day of each subsequent year.

  1. (a)

    Find the amount that Jeremy will have in the current account at the end of the year 2025 in terms of XX.

  2. (b)

    Find a simplified expression for the amount that Jeremy will have in the current account at the end of the nnth year.

  3. (c)

    Let X=8000X = 8000 for the remaining parts of the question.

    Jeremy plans to buy a car which costs $170 000.

    In which year will Jeremy have sufficient money in the current account to buy the car? Explain whether this occurs on the first or last day of the year.

  4. (d)

    Car financing loans are based on simple interest on the principal amount loaned. A bank offers a car loan at an annual interest rate of 2.8% for mm years to be repaid by equal instalments at the end of every year.

    If Jeremy takes a loan of $170 000, show that the annual instalment will be 1m(170000+4760m)\frac{1}{m}(170000 + 4760m).

  5. (e)

    Jeremy is considering the option to take this car loan for 10 years so that he can own the car earlier. From year 2025, instead of putting $8000 into the current account at the beginning of every year, he will use it to partially pay the annual instalment. The rest of the annual instalment will be deducted from the current account. Justify whether this arrangement is possible.

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Worked solution

10(a)

1. Calculate the amount in the account at the end of 2024 (the first year).

Amount at the end of 2024=100000×(1+3.2100)=100000×1.032=103200\text{Amount at the end of 2024} = 100000 \times \left(1 + \frac{3.2}{100}\right) = 100000 \times 1.032 = 103200

2. Form an expression for the amount at the end of 2025 by adding $X$ and applying interest again.

Amount at the beginning of 2025=103200+X\text{Amount at the beginning of 2025} = 103200 + X
Amount at the end of 2025=(103200+X)×1.032\text{Amount at the end of 2025} = (103200 + X) \times 1.032
=106502.4+1.032X= 106502.4 + 1.032X

Answer

106502.4+1.032X106502.4 + 1.032X

10(b)

1. Formulate the pattern for the total amount at the end of the $n$th year.

Let AnA_n be the amount at the end of year nn.
End of Year 1 (2024): A1=100000(1.032)A_1 = 100000(1.032)
End of Year 2 (2025): A2=100000(1.032)2+X(1.032)A_2 = 100000(1.032)^2 + X(1.032)
End of Year 3 (2026): A3=100000(1.032)3+X(1.032)2+X(1.032)A_3 = 100000(1.032)^3 + X(1.032)^2 + X(1.032)
Following this pattern, by the end of the nnth year:
An=100000(1.032)n+X(1.032n−1)+X(1.032n−2)+⋯+X(1.032)A_n = 100000(1.032)^n + X(1.032^{n-1}) + X(1.032^{n-2}) + \dots + X(1.032)

2. Identify the geometric series formed by the $X$ contributions and state its parameters.

The sequence of terms involving XX forms a geometric series:
X(1.032)+X(1.0322)+⋯+X(1.032n−1)X(1.032) + X(1.032^2) + \dots + X(1.032^{n-1})
First term, a=1.032Xa = 1.032X
Common ratio, r=1.032r = 1.032
Number of terms N=n−1N = n - 1

3. Apply the sum formula for a geometric progression and simplify the final expression.

Using the sum formula SN=a(rN−1)r−1S_N = \frac{a(r^N-1)}{r-1}:
Sum of XX terms =1.032X(1.032n−1−1)1.032−1= \frac{1.032X(1.032^{n-1} - 1)}{1.032 - 1}
=1.032X0.032(1.032n−1−1)= \frac{1.032X}{0.032}(1.032^{n-1} - 1)
=32.25X(1.032n−1−1)= 32.25X(1.032^{n-1} - 1)

Combining this with the initial investment's compounded amount gives the total amount AnA_n:
An=100000(1.032)n+32.25X(1.032n−1−1)A_n = 100000(1.032)^n + 32.25X(1.032^{n-1} - 1)

Answer

100000(1.032)n+32.25X(1.032n−1−1)100000(1.032)^n + 32.25X(1.032^{n-1} - 1)

10(c)

1. Find an expression for the amount at the end of the $n$-th year using $X = 8000$.

Using the expression for the sum from part (b), where the interest multiplier is 1.0321.032:
Sn=100 000(1.032)n+8000(1.032)(1.032n−1−1)1.032−1S_n = 100\,000(1.032)^n + \frac{8000(1.032)(1.032^{n-1} - 1)}{1.032 - 1}
Sn=100 000(1.032)n+258 000(1.032n−1−1)S_n = 100\,000(1.032)^n + 258\,000(1.032^{n-1} - 1)
Sn=100 000(1.032)n+250 000(1.032)n−258 000S_n = 100\,000(1.032)^n + 250\,000(1.032)^n - 258\,000
Sn=350 000(1.032)n−258 000S_n = 350\,000(1.032)^n - 258\,000

2. Solve for the year when the end-of-year balance reaches $170,000.

Set Sn≥170 000S_n \ge 170\,000:
350 000(1.032)n−258 000≥170 000350\,000(1.032)^n - 258\,000 \ge 170\,000
350 000(1.032)n≥428 000350\,000(1.032)^n \ge 428\,000
1.032n≥428350≈1.222861.032^n \ge \frac{428}{350} \approx 1.22286
n≥ln⁡(1.22286)ln⁡(1.032)≈6.386n \ge \frac{\ln(1.22286)}{\ln(1.032)} \approx 6.386
This means at the end of year 6, the amount is less than $170 000\char36{}170\,000.

3. Check the amount on the first day of the following year to conclude the exact year and day.

Amount at the end of year 6 (n=6n=6):
S6=350 000(1.032)6−258 000≈164 810.96S_6 = 350\,000(1.032)^6 - 258\,000 \approx 164\,810.96

Amount on the first day of year 7 (after depositing the additional $8000\char36{}8000):
164 810.96+8000=172 810.96164\,810.96 + 8000 = 172\,810.96

Since $172 810.96≥$170 000\char36{}172\,810.96 \ge \char36{}170\,000, Jeremy will have enough money on the first day of year 7.
Since year 1 is 2024, year 7 is 2024+7−1=20302024 + 7 - 1 = 2030.

Answer

Year 2030, on the first day.

10(d)

1. Calculate the total simple interest over $m$ years

Total Interest=170000×0.028×m=4760m\text{Total Interest} = 170000 \times 0.028 \times m = 4760m

2. Calculate the annual instalment

Total Repayment Amount=170000+4760m\text{Total Repayment Amount} = 170000 + 4760m

Annual instalment=170000+4760mm=1m(170000+4760m)(Shown)\text{Annual instalment} = \frac{170000 + 4760m}{m} = \frac{1}{m}(170000 + 4760m) \quad \text{(Shown)}

Answer

1m(170000+4760m)\frac{1}{m}(170000 + 4760m)

10(e)

1. Calculate the annual instalment for the car loan and the annual deduction from the current account.

Substitute m=10m = 10 into the formula from part (d):
Annual instalment=110(170000+4760(10))=21760010=$21760\text{Annual instalment} = \frac{1}{10}(170000 + 4760(10)) = \frac{217600}{10} = \$21760

Since Jeremy uses $8000\char36{}8000 to partially pay this instalment, the amount to be deducted from his current account at the end of each year is:
Annual deduction=21760−8000=$13760\text{Annual deduction} = 21760 - 8000 = \$13760

2. Formulate the future value of the current account after the 10-year loan period.

Assuming the loan is taken at the start of 2025, the balance at the end of 2024 before deductions is:
Balance at end of 2024=100000×1.032=$103200\text{Balance at end of 2024} = 100000 \times 1.032 = \$103200

For the next 10 years (end of 2025 to end of 2034), this initial sum accrues compound interest, while $13760\char36{}13760 is deducted at the end of each year.
Let AA be the final balance at the end of 2034:
A=103200(1.032)10−[13760(1.032)9+13760(1.032)8+⋯+13760(1.032)0]A = 103200(1.032)^{10} - \left[ 13760(1.032)^9 + 13760(1.032)^8 + \dots + 13760(1.032)^0 \right]

The deductions form a geometric progression with a=13760a = 13760, r=1.032r = 1.032, and n=10n = 10. Using the sum of a geometric series:
A=103200(1.032)10−13760(1.03210−11.032−1)A = 103200(1.032)^{10} - 13760 \left( \frac{1.032^{10} - 1}{1.032 - 1} \right)

3. Evaluate the balance to justify whether the arrangement is possible.

A=141409.19−13760(1.370244−10.032)A = 141409.19 - 13760 \left( \frac{1.370244 - 1}{0.032} \right)
A=141409.19−159204.98A = 141409.19 - 159204.98
A=−17795.79A = -17795.79

Since the projected final balance is negative (the accumulated value of the deductions exceeds the accumulated value of the initial deposit), the current account will not have sufficient funds to sustain the deductions for the full 10 years. Therefore, this arrangement is not possible.

Answer

The arrangement is not possible as the current account balance will fall short by $17 795.78\char36{}17\,795.78 by the end of the 10 years.

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