Algebra II : Interest Equations

Study concepts, example questions & explanations for Algebra II

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Example Questions

Example Question #1 : Interest Equations

Catherine invests $3500 in an investment account. The account earns 10% interest, compounded quarterly. After 5 years, how much money will she have?

Possible Answers:

\(\displaystyle \$5,250.00\)

\(\displaystyle \$3,850.00\)

\(\displaystyle \$5,735.15\)

\(\displaystyle \$6,283.78\)

\(\displaystyle \$5,636.79\)

Correct answer:

\(\displaystyle \$5,735.15\)

Explanation:

The formula for calculating the future value of an interest earning account is

\(\displaystyle FV = PV(1+\frac{r}{n})^{tn}\),

where

\(\displaystyle FV\)= future value,

\(\displaystyle PV\)= present value,

\(\displaystyle r\)= annual interest rate,

\(\displaystyle n\)= number of times the interest is compounded per year, and

\(\displaystyle t\)= the number of years that have passed.

The problem asks for the amount of money in the account after 5 years, with 10% interested compounded four times per year (quarterly).

Plug in the given quantities and simplify:

\(\displaystyle FV=$3500 (1+.025)^{5\cdot 4}\)

\(\displaystyle FV=3500(1.025)^{20}\)

\(\displaystyle {\color{Red} FV=\$5,735.15}\)

Example Question #1 : Interest Equations

Felicia put money in a saving account with a 5% interest rate, compounded annually. After five years, she had $10,000. How much was her initial investment?

Possible Answers:

\(\displaystyle 5,639.27\)

\(\displaystyle 8,798.40\)

\(\displaystyle 7,835.26\)

\(\displaystyle 5,367.18\)

\(\displaystyle $6,349.25\)

Correct answer:

\(\displaystyle 7,835.26\)

Explanation:

The formula for finding the future value of an investment is

\(\displaystyle FV=PV(1+r)^n\),

where

\(\displaystyle FV\)= future value,

\(\displaystyle PV\)= present value,

\(\displaystyle r\)= interest rate, and

\(\displaystyle n\)= number of times interest is compounded.

Plug in the given numbers and solve for the present value:

\(\displaystyle 10,000=PV(1+.05)^5\)

\(\displaystyle PV=\frac{10,000}{1.05^5}\)

\(\displaystyle {\color{Red} PV=7,835.26}\)

Example Question #1 : Interest Equations

Jamie deposits $5000 into an account at ABC bank. The account will earn a 4% interest rate compounded yearly. Jamie would like to withdraw the accumulated amount after 5 years and close the account. How much money would Jamie withdraw after 5 years? (Round your answer to the nearest dollar)

Possible Answers:

\(\displaystyle 6200\)

\(\displaystyle 6083\)

\(\displaystyle 5849\)

\(\displaystyle 6078\)

\(\displaystyle 6000\)

Correct answer:

\(\displaystyle 6083\)

Explanation:

Initial amount = 5000

The account earns 4% compounded yearly ===> Each $1.00 will grow into $1.04.

Growth rate = 1.04

Jamie will withdraw the money after 5 years. Since the interest is compounded yearly, the number of periods is equal to the number of years the money will be in the account.

number of periods = 5

From the above information, we can calculate the amount accumulated (or final amount) after 5 years using the following formula:

final amount = initial amount * (growth rate)number of periods

\(\displaystyle 5000\times(1.04)^{5}= 6083.2645\approx 6083\)

Example Question #1 : Interest Equations

Round the answer to two decimals.

Anthony put \(\displaystyle \$6\),\(\displaystyle 000\) in his savings account today. The bank pays interest of \(\displaystyle 3\%\) every year.

How much does he have in his savings account after \(\displaystyle 15\) years?

Possible Answers:

\(\displaystyle \$8,875.00\)

\(\displaystyle \$9,890.76\)

\(\displaystyle \$6,000\)

\(\displaystyle \$9,347.80\)

\(\displaystyle \$6,180.00\)

Correct answer:

\(\displaystyle \$9,347.80\)

Explanation:

The formula for computing interest is:

Beginning Amount x ((1 + rate)^number of years) = Ending Amount After number of years

\(\displaystyle E=B(1+r)^t\)

Make sure to convert the rate from percent to number: 3% = 0.03

\(\displaystyle E=6000 (1+0.03)^{15}\)

\(\displaystyle =6000(1.5580)\)

\(\displaystyle =9347.80\)

So the answer is \(\displaystyle \$9,347.80\)

Example Question #1 : Interest Equations

For coninuous compound interest:

 

\(\displaystyle P = Ce^{rt}\)

 

Where

\(\displaystyle P = \text{ final principal}\)

\(\displaystyle C = \text{ initial deposit}\)

\(\displaystyle r = \text{ interest rate}\)

\(\displaystyle t =\text{ number of years of investment}\)

If an initial deposit of \(\displaystyle \$250\) is continuously compounded at a rate of \(\displaystyle 6\%\) for \(\displaystyle 3\) years, what will be the final principal value to the nearest dollar?

Possible Answers:

\(\displaystyle \$1512\)

\(\displaystyle \$299\)

\(\displaystyle \$359\)

\(\displaystyle \$282\)

None of the other answers.

Correct answer:

\(\displaystyle \$299\)

Explanation:

Using the equation for continuous compound interest and the given information, we get 

\(\displaystyle P = Ce^{rt}\)

     \(\displaystyle P = \text{ final principal}\)

     \(\displaystyle C = 250\)

      \(\displaystyle r = \text{ 0.06}\)

      \(\displaystyle t =3\)

 

\(\displaystyle P = 250e^{\left ( .06\cdot 3\right )} =250e^{.18} = 250 \cdot 1.1972 = 299\)

Example Question #1 : Interest Equations

Remember         \(\displaystyle A = P(1+\frac{r}{n})^{nt}\)

If an account has a starting principle P = $5,000, an interest rate r = 12% or 0.12, compounded annually, how much money should there be after five years? Assume no money has been added or taken out of the account since it was opened. 

Possible Answers:

\(\displaystyle \$6162.37\)

\(\displaystyle \$6,189.78\)

\(\displaystyle \$8,811.71\)

\(\displaystyle \$7,174.29\)

\(\displaystyle \$10,212.13\)

Correct answer:

\(\displaystyle \$8,811.71\)

Explanation:

\(\displaystyle A = P(1+\frac{r}{n})^{nt}\)   is the compound interest formula where

P = Initial deposit = 5000

r = Interest rate = 0.12

n = Number of times interest is compounded per year = 1

t = Number of years that have passed = 5

\(\displaystyle 5000(1+.12)^5\)  \(\displaystyle =5000(1.762341683) = $8811.7084...\)

Round to the nearest cent or hundredth is \(\displaystyle \$8811.71\).

Example Question #1 : Interest Equations

Julio invests $5000 into an account with a 2.5% interest rate, compounded quarterly. What is his account balance after 1 year (rounded to the nearest cent)?

Possible Answers:

\(\displaystyle \$5126.18\)

\(\displaystyle \$5519.06\)

\(\displaystyle \$34864.50\)

\(\displaystyle \$5126.17\)

Correct answer:

\(\displaystyle \$5126.18\)

Explanation:

To determine Julio's account balance, we must use the interest formula given below:

\(\displaystyle P(1+\frac{r}{n})^{nt}\)

where P is his principal (initial) investment, r is the interest rate (as a decimal), n is the number of times the interest is compounded, and t is the amount of time elapsed.

Plugging in all of our given information into the above formula - knowing that quarterly means four times a year - we get

\(\displaystyle 5000(1+\frac{0.025}{4})^{4\cdot 1}=5126.1767...\approx5126.18\)

Example Question #1 : Applying Exponents

Martisha invests $2000 into an account with continuously compounded interest. The account has an interest rate of 2.5%. Find the balance of the account after 2 years, rounded to the nearest cent.

Possible Answers:

\(\displaystyle \$2050.63\)

\(\displaystyle \$2102.54\)

\(\displaystyle \$296826.32\)

\(\displaystyle \$3297.44\)

Correct answer:

\(\displaystyle \$2102.54\)

Explanation:

To find the balance, B, of a continuously compounded interest account after a certain amount of time, we must use the following formula:

\(\displaystyle B=Pe^{rt}\), where P is the initial investment, r is the interest rate (as a decimal), and t is the amount of time being considered.

Plugging in all of the given information, we get

\(\displaystyle B=2000e^{0.025\cdot 2}=2102.542193\)

which rounded becomes $2102.54

Example Question #2 : Interest Equations

How long will it take for Nikki to triple her initial investment into a continuously compounded interest account with an interest rate of 1.9%?

Possible Answers:

We are not given enough information to solve the problem

0.214 years

57.82 years

0.5782 years

Correct answer:

57.82 years

Explanation:

The formula to find the balance, B, of a continuously compounded interest account with interest rate, r, after a certain time, t, is given by

\(\displaystyle B=Pe^{rt}\)

To solve this problem, we need to know only the initial investment (P), our final balance (three times P) and the interest rate (expressed as a decimal), 0.019.

Plugging in our known information into the formula for continuously compounded interest, we get

\(\displaystyle 3P=Pe^{0.019t}\)

We now solve for t:

\(\displaystyle 3=e^{0.019t}\)

\(\displaystyle \ln(3)=\ln(e^{0.019t})\)

Exponentiating both sides allows us to get rid of the exponential:

\(\displaystyle \ln3=0.019t\)

\(\displaystyle t=57.82\)

Example Question #311 : Exponents

Sheila wants to double her initial investment into a compounded interest account, with an interest rate of 4%. How long will this take, if the interest is compounded annually?

Possible Answers:

0.52 years

1.923 years

0.0565 years

17.67 years

Correct answer:

17.67 years

Explanation:

To determine the amount of time needed to double the initial investment - P - into a compound interest account, we simply plug in our given information into the formula:

\(\displaystyle B=P(1+\frac{r}{n})^{nt}\)

where B is the balance, P is the initial investment, r is the interest rate (as a decimal), n is the number of times the interest is compounded, and t is the time elapsed.

Now, because we are doubling P, our balance B becomes two times P:

\(\displaystyle 2P=P(1+\frac{0.04}{1})^{1\cdot t}\)

Now, we can solve for P:

\(\displaystyle 2=1.04^t\)

To bring the time variable down from being an exponent, we take the logarithm of both sides (common or natural):

\(\displaystyle \ln2=t\cdot \ln1.04\)

\(\displaystyle t=17.67\)

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