# Simple and Compound Interest

We have to work with money every day. While balancing your checkbook or calculating your monthly expenditures on espresso requires only arithmetic, when we start saving, planning for retirement, or need a loan, we need more mathematics.### Learning Objectives

The leaning objectives for this section include:- Calculate one-time simple interest, and simple interest over time
- Determine APY given an interest scenario
- Calculate compound interest

## Simple Interest

Discussing interest starts with the**principal**, or amount your account starts with. This could be a starting investment, or the starting amount of a loan. Interest, in its most simple form, is calculated as a percent of the principal. For example, if you borrowed $100 from a friend and agree to repay it with 5% interest, then the amount of interest you would pay would just be 5% of 100: $100(0.05) = $5. The total amount you would repay would be $105, the original principal plus the interest.

### Simple One-time Interest

[latex-display]\begin{align}&I={{P}_{0}}r\\&A={{P}_{0}}+I={{P}_{0}}+{{P}_{0}}r={{P}_{0}}(1+r)\\\end{align}[/latex-display]*I*is the interest*A*is the end amount: principal plus interest- [latex]\begin{align}{{P}_{0}}\\\end{align}[/latex] is the principal (starting amount)
*r*is the interest rate (in decimal form. Example: 5% = 0.05)

### Examples

A friend asks to borrow $300 and agrees to repay it in 30 days with 3% interest. How much interest will you earn?Answer:

[latex]\begin{align}{{P}_{0}}\\\end{align}[/latex] = $300 | the principal |

r = 0.03 |
3% rate |

I = $300(0.03) = $9. |
You will earn $9 interest. |

### Exercises

Suppose your city is building a new park, and issues bonds to raise the money to build it. You obtain a $1,000 bond that pays 5% interest annually that matures in 5 years. How much interest will you earn?Answer: Each year, you would earn 5% interest: $1000(0.05) = $50 in interest. So over the course of five years, you would earn a total of $250 in interest. When the bond matures, you would receive back the $1,000 you originally paid, leaving you with a total of $1,250.

Further explanation about solving this example can be seen here. https://youtu.be/rNOEYPCnGwg### Simple Interest over Time

[latex-display]\begin{align}&I={{P}_{0}}rt\\&A={{P}_{0}}+I={{P}_{0}}+{{P}_{0}}rt={{P}_{0}}(1+rt)\\\end{align}[/latex-display]*I*is the interest*A*is the end amount: principal plus interest- [latex]\begin{align}{{P}_{0}}\\\end{align}[/latex] is the principal (starting amount)
*r*is the interest rate in decimal form*t*is time

### APR – Annual Percentage Rate

Interest rates are usually given as an**annual percentage rate (APR)**– the total interest that will be paid in the year. If the interest is paid in smaller time increments, the APR will be divided up. For example, a 6% APR paid monthly would be divided into twelve 0.5% payments. [latex-display]6\div{12}=0.5[/latex-display] A 4% annual rate paid quarterly would be divided into four 1% payments. [latex-display]4\div{4}=1[/latex-display]

### Example

Treasury Notes (T-notes) are bonds issued by the federal government to cover its expenses. Suppose you obtain a $1,000 T-note with a 4% annual rate, paid semi-annually, with a maturity in 4 years. How much interest will you earn?Answer: Since interest is being paid semi-annually (twice a year), the 4% interest will be divided into two 2% payments.

[latex]\begin{align}{{P}_{0}}\\\end{align}[/latex] = $1000 | the principal |

r = 0.02 |
2% rate per half-year |

t = 8 |
4 years = 8 half-years |

I = $1000(0.02)(8) = $160. |
You will earn $160 interest total over the four years. |

### Try It Now

## Compound Interest

With simple interest, we were assuming that we pocketed the interest when we received it. In a standard bank account, any interest we earn is automatically added to our balance, and we earn interest on that interest in future years. This reinvestment of interest is called**compounding**. Suppose that we deposit $1000 in a bank account offering 3% interest, compounded monthly. How will our money grow? The 3% interest is an annual percentage rate (APR) – the total interest to be paid during the year. Since interest is being paid monthly, each month, we will earn [latex]\frac{3%}{12}[/latex]= 0.25% per month. In the first month,

*P*= $1000_{0}*r*= 0.0025 (0.25%)*I*= $1000 (0.0025) = $2.50*A*= $1000 + $2.50 = $1002.50

*P*= $1002.50_{0}*I*= $1002.50 (0.0025) = $2.51 (rounded)*A*= $1002.50 + $2.51 = $1005.01

**compounding**interest gives us. Calculating out a few more months gives the following:

Month |
Starting balance |
Interest earned |
Ending Balance |

1 | 1000.00 | 2.50 | 1002.50 |

2 | 1002.50 | 2.51 | 1005.01 |

3 | 1005.01 | 2.51 | 1007.52 |

4 | 1007.52 | 2.52 | 1010.04 |

5 | 1010.04 | 2.53 | 1012.57 |

6 | 1012.57 | 2.53 | 1015.10 |

7 | 1015.10 | 2.54 | 1017.64 |

8 | 1017.64 | 2.54 | 1020.18 |

9 | 1020.18 | 2.55 | 1022.73 |

10 | 1022.73 | 2.56 | 1025.29 |

11 | 1025.29 | 2.56 | 1027.85 |

12 | 1027.85 | 2.57 | 1030.42 |

*P*represents the amount of money after

_{m }*m*months, then we could write the recursive equation:

*P*= $1000

_{0}*P*= (1+0.0025)

_{m}*P*You probably recognize this as the recursive form of exponential growth. If not, we go through the steps to build an explicit equation for the growth in the next example.

_{m-1}### Example

Build an explicit equation for the growth of $1000 deposited in a bank account offering 3% interest, compounded monthly.Answer:

*P*= $1000_{0}*P*= 1.0025_{1}*P*= 1.0025 (1000)_{0}*P*= 1.0025_{2}*P*= 1.0025 (1.0025 (1000)) = 1.0025 2(1000)_{1}*P*= 1.0025_{3}*P*= 1.0025 (1.00252(1000)) = 1.00253(1000)_{2}*P*= 1.0025_{4}*P*= 1.0025 (1.00253(1000)) = 1.00254(1000)_{3}

*P*= (1.0025)_{m}^{m}($1000)

*P*, the starting amount. We found 1.0025 by adding one to the growth rate divided by 12, since we were compounding 12 times per year. Generalizing our result, we could write [latex-display]{{P}_{m}}={{P}_{0}}{{\left(1+\frac{r}{k}\right)}^{m}}[/latex-display] In this formula:

_{0}*m*is the number of compounding periods (months in our example)*r*is the annual interest rate*k*is the number of compounds per year.

*N*is the number of years, then

*m = N k*. Making this change gives us the standard formula for compound interest.

### Compound Interest

[latex-display]P_{N}=P_{0}\left(1+\frac{r}{k}\right)^{Nk}[/latex-display]*P*is the balance in the account after_{N}*N*years.*P*is the starting balance of the account (also called initial deposit, or principal)_{0 }*r*is the annual interest rate in decimal form*k*is the number of compounding periods in one year- If the compounding is done annually (once a year),
*k*= 1. - If the compounding is done quarterly,
*k*= 4. - If the compounding is done monthly,
*k*= 12. - If the compounding is done daily,
*k*= 365.

- If the compounding is done annually (once a year),

*The most important thing to remember about using this formula is that it assumes that we put money in the account once and let it sit there earning interest.*### Example

A certificate of deposit (CD) is a savings instrument that many banks offer. It usually gives a higher interest rate, but you cannot access your investment for a specified length of time. Suppose you deposit $3000 in a CD paying 6% interest, compounded monthly. How much will you have in the account after 20 years?Answer: In this example,

P = $3000_{0} |
the initial deposit |

r = 0.06 |
6% annual rate |

k = 12 |
12 months in 1 year |

N = 20 |
since we’re looking for how much we’ll have after 20 years |

Years | Simple Interest ($15 per month) | 6% compounded monthly = 0.5% each month. |

5 | $3900 | $4046.55 |

10 | $4800 | $5458.19 |

15 | $5700 | $7362.28 |

20 | $6600 | $9930.61 |

25 | $7500 | $13394.91 |

30 | $8400 | $18067.73 |

35 | $9300 | $24370.65 |

### Try It Now

### Rounding

It is important to be very careful about rounding when calculating things with exponents. In general, you want to keep as many decimals during calculations as you can. Be sure to**keep at least 3 significant digits**(numbers after any leading zeros). Rounding 0.00012345 to 0.000123 will usually give you a “close enough” answer, but keeping more digits is always better.

### Example

To see why not over-rounding is so important, suppose you were investing $1000 at 5% interest compounded monthly for 30 years.P = $1000_{0} |
the initial deposit |

r = 0.05 |
5% |

k = 12 |
12 months in 1 year |

N = 30 |
since we’re looking for the amount after 30 years |

*r/k*, we find 0.05/12 = 0.00416666666667 Here is the effect of rounding this to different values:

r/k rounded to: |
Gives P30 to be: |
Error |

0.004 | $4208.59 | $259.15 |

0.0042 | $4521.45 | $53.71 |

0.00417 | $4473.09 | $5.35 |

0.004167 | $4468.28 | $0.54 |

0.0041667 | $4467.80 | $0.06 |

no rounding | $4467.74 |

### Using your calculator

In many cases, you can avoid rounding completely by how you enter things in your calculator. For example, in the example above, we needed to calculate [latex]{{P}_{30}}=1000{{\left(1+\frac{0.05}{12}\right)}^{12\times30}}[/latex] We can quickly calculate 12×30 = 360, giving [latex]{{P}_{30}}=1000{{\left(1+\frac{0.05}{12}\right)}^{360}}[/latex]. Now we can use the calculator.Type this |
Calculator shows |

0.05 ÷ 12 = . | 0.00416666666667 |

+ 1 = . | 1.00416666666667 |

yx 360 = . | 4.46774431400613 |

× 1000 = . | 4467.74431400613 |

### Using your calculator continued

The previous steps were assuming you have a “one operation at a time” calculator; a more advanced calculator will often allow you to type in the entire expression to be evaluated. If you have a calculator like this, you will probably just need to enter: 1000 × ( 1 + 0.05 ÷ 12 ) y^{x}360 =

## Solving For Time

### Examples

If you invest $2000 at 6% compounded monthly, how long will it take the account to double in value?Answer: This is a compound interest problem, since we are depositing money once and allowing it to grow. In this problem,

P = $2000_{0} |
the initial deposit |

r = 0.06 |
6% annual rate |

k = 12 |
12 months in 1 year |

*N*so that

*P*= 4000. To solve this, we set our equation for

_{N}*P*equal to 4000.

_{N}[latex]4000=2000{{\left(1+\frac{0.06}{12}\right)}^{N\times12}}[/latex] Divide both sides by 2000

[latex]2={{\left(1.005\right)}^{12N}}[/latex] To solve for the exponent, take the log of both sides

[latex]\log\left(2\right)=\log\left({{\left(1.005\right)}^{12N}}\right)[/latex] Use the exponent property of logs on the right side

[latex]\log\left(2\right)=12N\log\left(1.005\right)[/latex] Now we can divide both sides by 12log(1.005)

[latex]\frac{\log\left(2\right)}{12\log\left(1.005\right)}=N[/latex] Approximating this to a decimal

*N*= 11.581 It will take about 11.581 years for the account to double in value. Note that your answer may come out slightly differently if you had evaluated the logs to decimals and rounded during your calculations, but your answer should be close. For example if you rounded log(2) to 0.301 and log(1.005) to 0.00217, then your final answer would have been about 11.577 years. Get additional guidance for this example in the following: https://youtu.be/zHRTxtFiyxc

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