# GMAT Math : Calculating compound interest

## Example Questions

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Grandpa Jack wants to help his grandson, Little Jack, with college expenses. Little Jack is currently 3 years old. If Grandpa Jack invests $5,000 in a college savings account earning 5% compounded yearly, how much money will he have in 15 years when Little Jack is 18? Possible Answers: Between$10,000-$10,500 Between$10,500-$11,000 Between$9,000-$9,500 Between$11,000-$11,500 Between$9,500-$10,000 Correct answer: Between$10,000-$10,500 Explanation: To solve this, we can create an equation for the value based on time. So if we let t be the nmbers of years that have passed, we can create a function f(t) for the value in the savings account. We note that f(0) =5000. (We invest 5000 at time 0.) Next year, he will have 5% more than that. To find our total value at the end of the year, we multiply 5,000 * 1.05 = 5,250. f(1) = 5000(1.05)=5,250. At the end of year 2, we will have a 5% growth rate. In other words, f(2) = (1.05)* f(1). We can rewrite this as . We can begin to see the proper equation is . If we plug in t = 15, we will have our account balance at the end of 15 years. So, our answer is . ### Example Question #2 : Calculating Compound Interest Cherry invested dollars in a fund that paid 6% annual interest, compounded monthly. Which of the following represents the value, in dollars, of Cherry’s investment plus interest at the end of 3 years? Possible Answers: Correct answer: Explanation: The monthly rate is 3 years = 36 months According to the compound interest formula and here , , , so we can plug into the formula and get the value ### Example Question #3 : Calculating Compound Interest Scott wants to invest$1000 for 1 year. At Bank A, his investment will collect 3% interest compounded daily while at Bank B, his investment will collect 3.50% interest compounded monthly. Which bank offers a better return? How much more will he receive by choosing that bank over the other?

Explanation:

Calculate the total amount from each bank using the following formula:

Bank A:

Bank B:

### Example Question #4 : Calculating Compound Interest

Bryan invests $8,000 in both a savings account that pays 3% simple interest annually and a certificate of deposit that pays 8% simple interest anually. After the first year, Bryan has earned a total of$365.00 from these investments. How much did Bryan invest in the certificate of deposit?

Explanation:

Let  be the amount Bryan invested in the certificate of deposit. Then he deposited  in a savings account. 8% of the amount in the certificate of deposit is , and 3% of the amount in the savings account is ; add these interest amounts to get $365.00. Therefore, we can set up and solve the equation: ### Example Question #5 : Calculating Compound Interest Barry invests$9000 in corporate bonds at 8% annual interest, compounded quarterly. At the end of the year, how much interest has his investment earned?

Explanation:

Use the compound interest formula

substituting  (principal, or amount invested),  (decimal equivalent of the 8% interest rate),  (four quarters per year),  (one year).

Subtract 9,000 from this figure - the interest earned is $741.89 ### Example Question #6 : Calculating Compound Interest Tom deposits his$10,000 inheritance in a savings account with a 4% annual interest rate, compounded quarterly. He leaves it there untouched for six months, after which he withdraws $5,000. He leaves the remainder untouched for another six months. How much interest has Tom earned on the inheritance after one year? Possible Answers: Correct answer: Explanation: Since in each case the interest is compounded quarterly, the annual interest rate of 4% is divided by 4 to get 1%, the effective quarterly interest rate. The$10,000 remains in the savings account six months, or two quarters, so 1% is added twice - equivalently, the $10,000 is multiplied by 1.01 twice:$5,000 is withdrawn from the savings account, leaving

This money is untouched for six months, or two quarters, so again, we multiply by 1.01 twice:

First quarter:

Second quarter:

Third quarter:

Fourth quarter:

The thriteenth payment, with which Gary will pay off the loan, will be $913.16. ### Example Question #8 : Calculating Compound Interest Jessica deposits$5,000 in a savings account at 6% interest. The interest is compounded monthly. How much will she have in her savings account after 5 years?

None of the other answers are correct.

Explanation:

where  is the principal,  is the number of times per year interest is compounded,  is the time in years, and  is the interest rate.

We have