Math Problem Statement

interest rate is wrong

You invest $20,000 in an account where the interest is continuously compounding. If in 10 years you have $30,000, find the interest (growth) rate and the doubling time.

Solution

To solve for the correct interest rate with continuous compounding, we can use the formula:

A=PertA = P e^{rt}

where:

  • AA is the amount after time tt,
  • PP is the initial principal,
  • rr is the interest rate (growth rate),
  • tt is the time in years,
  • ee is the base of the natural logarithm.

Given Data

  • Initial amount, P=20,000P = 20,000
  • Amount after 10 years, A=30,000A = 30,000
  • Time, t=10t = 10 years
  1. Set up the equation:
    30,000=20,000e10r30,000 = 20,000 \cdot e^{10r}

  2. Solve for rr: Divide both sides by 20,000: 1.5=e10r1.5 = e^{10r}

    Take the natural logarithm of both sides: ln(1.5)=10r\ln(1.5) = 10r

    Solve for rr: r=ln(1.5)10r = \frac{\ln(1.5)}{10}

  3. Calculate rr to two decimal places.

Let me calculate this.The correct interest rate, rounded to two decimal places, is 4.05%.

Let me know if you'd like further details or if you have any questions.

Here are some related questions to explore:

  1. How does continuous compounding differ from annual compounding?
  2. How can the formula be modified if compounding occurs quarterly instead of continuously?
  3. What is the doubling time for a 4.05% continuously compounded rate?
  4. How would a change in the time period affect the final amount?
  5. What is the effect of increasing the initial investment on the final amount?

Tip: When working with continuously compounded interest, remember that small changes in the rate or time can significantly impact the final amount.

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Math Problem Analysis

Mathematical Concepts

Exponential Growth
Continuous Compounding
Logarithmic Functions

Formulas

A = P * e^(rt)
ln(A/P) = rt

Theorems

Properties of Natural Logarithms

Suitable Grade Level

Grades 10-12