Given the model [tex]A = P e^{rt}[/tex] or [tex]A = P\left(1 + \frac{r}{n}\right)^{nt}[/tex], where [tex]A[/tex] is the future value of [tex]P[/tex] dollars invested at an interest rate [tex]r[/tex] compounded continuously or [tex]n[/tex] times per year:

If you have \[tex]$100,000 in a retirement account, how long will it take for the money to grow to \$[/tex]1,000,000 if it grows by 7.5% compounded continuously?

Round your answer to the nearest year.

It will take approximately [tex]\square[/tex] years.



Answer :

Sure! Let's solve this step-by-step.

1. Identify the Parameters:
- Initial investment, [tex]\( P = \$100,000 \)[/tex]
- Future value, [tex]\( A = \$1,000,000 \)[/tex]
- Interest rate, [tex]\( r = 7.5\% \)[/tex] or [tex]\( 0.075 \)[/tex]
- We are using the formula for continuous compounding.

2. Use the Continuous Compounding Formula:
The formula for continuous compounding is:
[tex]\[ A = P e^{rt} \][/tex]

3. Rearrange to Solve for [tex]\( t \)[/tex]:
We need to find the time [tex]\( t \)[/tex], so we rearrange the formula to solve for [tex]\( t \)[/tex]:
[tex]\[ t = \frac{\ln(A/P)}{r} \][/tex]

4. Calculate the Natural Logarithm:
Substitute the values:
[tex]\[ t = \frac{\ln(1,000,000 / 100,000)}{0.075} \][/tex]
Simplify inside the logarithm:
[tex]\[ t = \frac{\ln(10)}{0.075} \][/tex]

5. Evaluate [tex]\( \ln(10) \)[/tex]:
The natural logarithm of 10 is approximately 2.302585.

6. Complete the Calculation:
[tex]\[ t = \frac{2.302585}{0.075} \][/tex]
[tex]\[ t \approx 30.701134573253945 \][/tex]

7. Round to the Nearest Year:
The nearest integer value of 30.701134573253945 is 31.

Therefore, it will take approximately 31 years for the money to grow from \[tex]$100,000 to \$[/tex]1,000,000 at an interest rate of 7.5% compounded continuously.

Final Answer:
It will take approximately 31 years.