Gianna is going to invest \( \$ 700 \) and leave it in an account for 14 years. Assuming the interest is compounded daily, what interest rate, to the nearest tenth of a percent, would be required in order for Gianna to end up with \( \$ 1,010 \) ?
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The Deep Dive
To solve this problem, we use the formula for compound interest: \[ A = P(1 + \frac{r}{n})^{nt} \] Where: - \( A \) = the amount of money accumulated after n years, including interest. - \( P \) = the principal amount (the initial amount of money). - \( r \) = the annual interest rate (in decimal). - \( n \) = the number of times that interest is compounded per year. - \( t \) = the number of years the money is invested. In this case: - \( A = 1010 \) - \( P = 700 \) - \( n = 365 \) (since interest is compounded daily) - \( t = 14 \) Now we will plug in the values and solve for \( r \): \[ 1010 = 700 \left(1 + \frac{r}{365}\right)^{365 \times 14} \] First, divide both sides by 700: \[ \frac{1010}{700} = \left(1 + \frac{r}{365}\right)^{5110} \] Which simplifies to: \[ 1.442857 = \left(1 + \frac{r}{365}\right)^{5110} \] Next, we take the natural logarithm of both sides: \[ \ln(1.442857) = 5110 \cdot \ln\left(1 + \frac{r}{365}\right) \] Now, calculate \( \ln(1.442857) \): \[ \ln(1.442857) \approx 0.366 \] So the equation becomes: \[ 0.366 = 5110 \cdot \ln\left(1 + \frac{r}{365}\right) \] Now, divide both sides by 5110: \[ \ln\left(1 + \frac{r}{365}\right) \approx \frac{0.366}{5110} \approx 0.0000717 \] Next, we exponentiate both sides: \[ 1 + \frac{r}{365} \approx e^{0.0000717} \approx 1.0000717 \] Subtract 1 from both sides: \[ \frac{r}{365} \approx 0.0000717 \] Multiply both sides by 365 to solve for \( r \): \[ r \approx 0.0262 \quad (\text{or } 2.62 \% \text{ as a percentage}) \] Finally, rounding to the nearest tenth of a percent, the required interest rate is approximately \( 2.6\% \) per year.
