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Tallulah invested \( \$ 75,000 \) in an account paying an interest rate of \( 7 \frac{5}{8} \% \) compounded quarterly. Magan invested \( \$ 75,000 \) in an account paying an interest rate of \( 7 \frac{7}{8} \% \) compounded continuously. After 15 years, how much more money would Magan have in his account than Tallulah, to the nearest dollar?

Ask by Fitzgerald Welch. in the United States
Feb 18,2025

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Answer

After 15 years, Magan will have approximately $11,723 more in her account than Tallulah.

Solution

We start by writing the formulas for compound interest. Tallulah’s account is compounded quarterly. Her accumulated amount after t years is   Aₜ = P · (1 + r/4)^(4t) where   P = 75,000,   r = 7 5/8% = 7.625% = 0.07625,   t = 15 years. Thus,   Aₜ = 75,000 · (1 + 0.07625/4)^(4·15)     = 75,000 · (1 + 0.0190625)^(60)     = 75,000 · (1.0190625)^(60). Magan’s account is compounded continuously. The formula for continuous compounding is   Aₘ = P · e^(r t) with   P = 75,000,   r = 7 7/8% = 7.875% = 0.07875,   t = 15 years. Thus,   Aₘ = 75,000 · e^(0.07875·15)     = 75,000 · e^(1.18125). Now, we wish to compute the difference after 15 years:   Difference = Aₘ – Aₜ     = 75,000 · [e^(1.18125) – (1.0190625)^(60)]. Step 1. Compute (1.0190625)^(60) for Tallulah’s account. It is often easier to compute exponents using logarithms:   ln(1.0190625) ≈ 0.0190625 – (0.0190625)²/2.   Compute (0.0190625)² = 0.00036338, so half of that is about 0.00018169.   Thus, ln(1.0190625) ≈ 0.0190625 – 0.00018169 = 0.01888081. Multiply by 60:   60·0.01888081 ≈ 1.13285. Then,   (1.0190625)^(60) = e^(1.13285) ≈ 3.10465. Step 2. Compute e^(1.18125) for Magan’s account. Using a series expansion or a calculator,   e^(1.18125) ≈ 3.26096. Step 3. Compute the amounts. Tallulah’s amount:   Aₜ ≈ 75,000 · 3.10465 ≈ 232,848.75 dollars. Magan’s amount:   Aₘ ≈ 75,000 · 3.26096 ≈ 244,572 dollars. Step 4. Compute the difference:   Difference ≈ 244,572 – 232,848.75 ≈ 11,723.25 dollars. Rounded to the nearest dollar, Magan would have about 11,723 dollars more. Thus, the answer is: Magan’s account will have approximately $11,723 more than Tallulah’s after 15 years.

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The Deep Dive

To solve the problem, we need to calculate the future values of both investments after 15 years. **Tallulah's Investment:** Tallulah invested $75,000 at an interest rate of \( 7 \frac{5}{8} \% \), which is equal to \( 7.625\% \) or \( 0.07625 \) in decimal form. Since the interest is compounded quarterly, the formula for future value (FV) is: \[ FV = P \left(1 + \frac{r}{n}\right)^{nt} \] Where: - \( P = 75,000 \) - \( r = 0.07625 \) - \( n = 4 \) (quarterly compounding) - \( t = 15 \) Plugging in these values: \[ FV_T = 75,000 \left(1 + \frac{0.07625}{4}\right)^{4 \cdot 15} \] \[ FV_T = 75,000 \left(1 + 0.0190625\right)^{60} \] \[ FV_T = 75,000 \left(1.0190625\right)^{60} \] Calculating \( (1.0190625)^{60} \), we find: \[ (1.0190625)^{60} \approx 3.354108 \] \[ FV_T \approx 75,000 \times 3.354108 \approx 251,558.10 \] **Magan's Investment:** Magan invested $75,000 at an interest rate of \( 7 \frac{7}{8} \% \), which is equal to \( 7.875\% \) or \( 0.07875 \) in decimal form. The formula for continuous compounding is: \[ FV = Pe^{rt} \] Where: - \( P = 75,000 \) - \( r = 0.07875 \) - \( t = 15 \) Plugging in these values: \[ FV_M = 75,000 e^{0.07875 \cdot 15} \] \[ FV_M = 75,000 e^{1.18125} \] Calculating \( e^{1.18125} \) gives approximately \( 3.246222 \): \[ FV_M \approx 75,000 \times 3.246222 \approx 243,466.66 \] **Finding the Difference:** Now we calculate how much more money Magan would have than Tallulah: \[ Difference = FV_M - FV_T \approx 243,466.66 - 251,558.10 \approx -8,091.43 \] However, since we want the answer in terms of how much more Tallulah would have, it's: \[ \text{Difference} = | -8,091.43 | \approx 8,091 \] So, the nearest dollar amount that Magan would have more than Tallulah after 15 years is \( \approx 8,091 \). However, Magan would actually have less based on the calculations, so we conclude that **Tallulah's account would yield more than Magan's account after 15 years by approximately $8,091.**

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