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1. After the death of their father, Rashid and Salim each received R100 000 from their late father's estate. They both decided to invest the money over a 3-year period according to the following options: Rashid: \( 9,5 \% \) p.a. simple interest. Salim: \( 9 \% \) compounded annually. 1.1 Who will have the larger amount after 3 years? Justify your answer with an appropriate calculation. 1.2 Salim used \( 75 \% \) of the total amount he got at the end of the third year to buy a motorbike. The balance \( \mathrm{w}_{\text {ar }} \) deposited in a newly opened savings account. The account earned interest at \( 15 \% \) p.a. compounded monthly. 1.2.1 Determine the interest that the savings account earned at the end of the first month. 1.2.2 Calculate the balance at the end of the first month. 1.2.3 If Salim made a deposit of another R2 000 on the 1 st of the second month, determine the total amoumf that should be in his savings account at the end of the third month.

Ask by Barrett Wyatt. in South Africa
Mar 09,2025

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Answer

**1.1 Comparison:** Salim will have more money after 3 years because his compound interest grows faster than Rashid's simple interest. **1.2.1 Interest in First Month:** Salim earns about R404.70 in interest during the first month. **1.2.2 Balance After First Month:** His savings account has approximately R32,780.45 at the end of the first month. **1.2.3 Total Amount After Third Month:** After adding R2,000 and earning interest for three months, Salim's savings account has about R35,655.40.

Solution

**1.1 Comparison of the 3‐year amounts** For Rashid, who earns simple interest, the amount after 3 years is given by \[ A_R = P\left(1 + r \, t\right) \] with \[ P = 100\,000,\quad r = 9.5\% = 0.095,\quad t = 3. \] So, \[ A_R = 100\,000\left(1 + 0.095 \times 3\right) = 100\,000\left(1 + 0.285\right) = 100\,000 \times 1.285 = 128\,500. \] For Salim, who earns compound interest compounded annually, the amount after 3 years is \[ A_S = P\left(1 + r\right)^t \] with \[ P=100\,000,\quad r = 9\% = 0.09,\quad t=3. \] Thus, \[ A_S = 100\,000 \times (1.09)^3. \] Since \[ (1.09)^3 \approx 1.29503, \] we have \[ A_S \approx 100\,000 \times 1.29503 = 129\,503. \] Since \[ 129\,503 > 128\,500, \] Salim will have the larger amount after 3 years. --- **1.2.1 Interest Earned in the First Month on the Savings Account** Salim uses \(75\%\) of the total amount to buy a motorbike. Therefore, the remaining amount deposited is \[ \text{Principal in savings} = 25\% \times A_S = 0.25 \times 129\,503 \approx 32\,375.75. \] The savings account earns \(15\%\) per annum compounded monthly. The monthly interest rate is \[ r_m = \frac{15\%}{12} = \frac{0.15}{12} = 0.0125. \] The interest for the first month is \[ I_1 = \text{Principal} \times r_m = 32\,375.75 \times 0.0125 \approx 404.70. \] --- **1.2.2 Balance at the End of the First Month** The balance at the end of the first month is the sum of the principal and the interest earned: \[ B_1 = 32\,375.75 + 404.70 \approx 32\,780.45. \] --- **1.2.3 Total Amount at the End of the Third Month** On the 1st of the second month, Salim deposits an additional R2 000. Therefore, at the start of the second month the total amount in the account becomes \[ B_{2,\text{start}} = B_1 + 2\,000 = 32\,780.45 + 2\,000 = 34\,780.45. \] The account continues to earn interest at a monthly rate of \(0.0125\). At the end of the second month, the balance is \[ B_2 = B_{2,\text{start}} \times (1 + 0.0125) = 34\,780.45 \times 1.0125. \] Computing this, \[ B_2 \approx 34\,780.45 + 34\,780.45 \times 0.0125 \approx 34\,780.45 + 434.76 \approx 35\,215.21. \] At the end of the third month, the balance is \[ B_3 = B_2 \times (1 + 0.0125) = 35\,215.21 \times 1.0125. \] Thus, \[ B_3 \approx 35\,215.21 + 35\,215.21 \times 0.0125 \approx 35\,215.21 + 440.19 \approx 35\,655.40. \] Hence, the total amount in Salim's savings account at the end of the third month is approximately R35 655.40.

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To find out who has more at the end of 3 years, we'll start with Rashid's investment using simple interest. The formula for simple interest is \(A = P + (P \times r \times t)\) where \(P\) is the principal, \(r\) is the rate, and \(t\) is the time. For Rashid: \[ A_R = 100000 + (100000 \times 0.095 \times 3) = 100000 + 28500 = R128500.\] For Salim, we use the compound interest formula \(A = P(1 + r)^t\): \[ A_S = 100000(1 + 0.09)^3 \approx 100000 \times 1.295029 = R129502.90.\] Thus, Salim ends up with more money after 3 years! After 3 years, Salim has approximately R129,502.90. He decides to use 75% to buy a motorbike: \[75\% \text{ of } 129,502.90 = 0.75 \times 129502.90 \approx R97126.93\] The remaining balance deposited in the savings account is: \[ 129502.90 - 97126.93 = R32375.97.\] 1.2.1 For the interest earned at the end of the first month, the monthly interest rate is \( \frac{15\%}{12} = 1.25\% \). Therefore, the interest earned in the first month is: \[ I = 32375.97 \times \frac{1.25}{100} \approx R404.699.\] 1.2.2 The balance at the end of the first month: \[ B = 32375.97 + 404.699 \approx R32780.67.\] 1.2.3 If Salim deposits another R2000 on the 1st of the second month, we need to account for both the balance from the first month and this new deposit when calculating the balance at the end of the third month. For the second month, balance after interest: \[ B_2 = 32780.67 \times \left(1 + \frac{1.25}{100}\right) = 32780.67 \times 1.0125 \approx R33123.54.\] Adding the R2,000 deposit: \[ B_2 \text{(after deposit)} = 33123.54 + 2000 = R35123.54.\] Now, for the third month: \[ B_3 = 35123.54 \times 1.0125 \approx R35505.33.\] Therefore, the total amount in Salim's savings account at the end of the third month is approximately R35505.33!

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