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How long will it take \(\$ 200\) to double if it earns the following rates? Compounding occurs once a year. a. \(7 \%\) b. \(10 \%\) c. \(18 \%\) d. \(100 \%\)

Short Answer

Expert verified
(a) ≈ 10.24 years, (b) ≈ 7.27 years, (c) ≈ 4.20 years, (d) 1 year.

Step by step solution

01

Understand the Problem

We need to determine the time it takes for an investment to double using different annual interest rates. The formula to use is derived from the compound interest formula. Our principal amount doubles, so if we start with \( P \), we want it to become \( 2P \).
02

Identify the Formula

For doubling investments, we use the formula \( A = P(1 + r)^t \). Here, \( A \) is the final amount (which will be \( 2P \)), \( P \) is the initial investment (\( 200 \)), \( r \) is the annual interest rate, and \( t \) is the time in years. We rearrange the formula to \( t = \frac{\ln(2)}{\ln(1 + r)} \).
03

Calculate Time for 7% Interest

For \( r = 0.07 \), the formula becomes \( t = \frac{\ln(2)}{\ln(1.07)} \). Calculate \( t \):1. \( \ln(2) \approx 0.693 \)2. \( \ln(1.07) \approx 0.0677 \)3. \( t = \frac{0.693}{0.0677} \approx 10.24 \)So, it will take approximately 10.24 years.
04

Calculate Time for 10% Interest

For \( r = 0.10 \), use \( t = \frac{\ln(2)}{\ln(1.10)} \). Calculate \( t \):1. \( \ln(2) \approx 0.693 \)2. \( \ln(1.10) \approx 0.0953 \)3. \( t = \frac{0.693}{0.0953} \approx 7.27 \)So, it will take approximately 7.27 years.
05

Calculate Time for 18% Interest

For \( r = 0.18 \), use \( t = \frac{\ln(2)}{\ln(1.18)} \). Calculate \( t \):1. \( \ln(2) \approx 0.693 \)2. \( \ln(1.18) \approx 0.1655 \)3. \( t = \frac{0.693}{0.1655} \approx 4.20 \)So, it will take approximately 4.20 years.
06

Calculate Time for 100% Interest

For \( r = 1 \), use \( t = \frac{\ln(2)}{\ln(2)} \). Calculate \( t \):1. \( \ln(2) \approx 0.693 \)2. \( \ln(2) \approx 0.693 \)3. \( t = \frac{0.693}{0.693} = 1 \)So, it will take exactly 1 year.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Compound Interest Formula
Compound interest makes your money grow at an accelerating rate. It's much like a snowball that accumulates more snow as it rolls downhill. The compound interest formula is essential for understanding how your investment grows over time. It is given as:
  • \( A = P(1 + r)^t \)
Here,
  • \( A \) is the future value of the investment.
  • \( P \) is the principal investment amount.
  • \( r \) is the annual interest rate in decimal.
  • \( t \) is the time the money is invested for in years.
Using this formula helps financial planners calculate growth accurately. By understanding this formula, you can predict how much your initial investment will be worth in the future.
Investment Doubling
Investment doubling refers to the time required for the initial investment to grow to twice its size using a certain interest rate. A quick way to estimate this is the "Rule of 72." This rule is a simple trick for mental math:
  • Divide 72 by the annual interest rate.
This approximation provides a handy way to foresee how soon your money could double under compounding. For example, for a 10% interest rate, divide 72 by 10, suggesting a doubling time of about 7.2 years. By relying on actual calculations and formulas, as in our exercise, you pinpoint more precise doubling periods.
Exponential Growth
Exponential growth is fascinating. It signifies increases at a constantly accelerating rate. In terms of finance, it depicts how investments grow faster over time as interest accrues on both the initial principal and accumulated interest.
This type of growth is beneficial in the context of compound interest, where each time interest is added to the principal, the base for further interest calculations increases.
The broader the timespan, the more pronounced this growth's effect becomes. Understanding exponential growth allows investors to appreciate why starting early makes a significant difference in their investment value over time.
Starting with compound interest investments as soon as possible guarantees higher cumulative growth over time.
Financial Calculations
Financial calculations are vital in personal finance and investment planning. They help in estimating the growth of funds under different scenarios and guiding sound decision-making.
  • Using formulas like the compound interest one, you can calculate future values, interest earned, and doubling times.
  • This ensures your investment strategy aligns with your financial goals.
While precise calculations can seem daunting at first, understanding the logic behind them equips you with tools for better financial planning. Calculators and software can assist you in these calculations, but knowledge of how they work provides an additional layer of understanding and control.
Take opportunities to practice these calculations, like determining how much you'll have after a set period given a fixed interest rate, to build confidence in managing your investments.

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Most popular questions from this chapter

You want to buy a car, and a local bank will lend you \(\$ 20,000 .\) The loan will be fully amortized over 5 years \((60\) months), and the nominal interest rate will be \(12 \%\) with interest paid monthly. What will be the monthly loan payment? What will be the loan's EAR?

It is now December \(31,2008(\mathrm{t}=0),\) and a jury just found in favor of a woman who sued the city for injuries sustained in a January 2007 accident. She requested recovery of lost wages plus \(\$ 100,000\) for pain and suffering plus \(\$ 20,000\) for legal expenses. Her doctor testified that she has been unable to work since the accident and that she will not be able to work in the future. She is now 62 , and the jury decided that she would have worked for another 3 years. She was scheduled to have earned \(\$ 34,000\) in 2007 . (To simplify this problem, assume that the entire annual salary amount would have been received on December 31,2007 .) Her employer testified that she probably would have received raises of \(3 \%\) per year. The actual payment will be made on December 31,2009 The judge stipulated that all dollar amounts are to be adjusted to a present value basis on December \(31,2009,\) using a \(7 \%\) annual interest rate and using compound, not simple, interest. Furthermore, he stipulated that the pain and suffering and legal expenses should be based on a December, \(31,2008,\) date. How large a check must the city write on December \(31,2009 ?\)

You want to buy a house within 3 years, and you are currently saving for the down payment. You plan to save \(\$ 5,000\) at the end of the first year, and you anticipate that your annual savings will increase by \(10 \%\) annually thereafter. Your expected annual return is \(7 \% .\) How much will you have for a down payment at the end of Year \(3 ?\)

Find the present values of these ordinary annuities. Discounting occurs once a year. a. \(\$ 400\) per year for 10 years at \(10 \%\) b. \(\$ 200\) per year for 5 years at \(5 \%\) c. \(\$ 400\) per year for 5 years at \(0 \%\) d. Rework Parts a, b, and c assuming they are annuities due.

Your father is 50 years old and will retire in 10 years. He expects to live for 25 years after he retires, until he is \(85 .\) He wants a fixed retirement income that has the same purchasing power at the time he retires as \(\$ 40,000\) has today. (The real value of his retirement income will decline annually after he retires.) His retirement income will begin the day he retires, 10 years from today, at which time he will receive 24 additional annual payments. Annual inflation is expected to be \(5 \%\). He currently has \(\$ 100,000\) saved, and he expects to earn \(8 \%\) annually on his savings. How much must he save during each of the next 10 years (end-of-year deposits) to meet his retirement goal?

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