# GED Math : Circumference

## Example Questions

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### Example Question #1 : Circumference

Give the circumference of a circle with radius 6. Round to the nearest tenth.

Possible Answers:

Correct answer:

Explanation:

The circumference of a circle can be found using this formula:

Where

Substitute  and use , since we are rounding:

Round this to .

### Example Question #2 : Circumference

A car whose tires have diameter 30 inches moves at a rate of 24 miles per hour. How many revolutions will each tire make in one minute (nearest whole number)?

Possible Answers:

Correct answer:

Explanation:

We will convert all linear distances to feet and all times to minutes.

The tires have diameter 30 inches, which is equal to  feet. Their circumference will be  times this, or  feet.

The car moves at a rate of 25 miles per hour, which is equivalent to

feet per minute.

The number of revolutions each tire makes is the distance traveled divided by the circumference of the tire, which is

.

Each tire revolves about 269 times in the course of one minute.

### Example Question #3 : Circumference

What is the circumference of a circle given the radius is 7?

Possible Answers:

Correct answer:

Explanation:

The equation of circumference of a circle can be found using . By substituting in our radius of 7 we can solve for C.

### Example Question #4 : Circumference

What is the circumference of a circle with a diameter of 10 inches?

Possible Answers:

Correct answer:

Explanation:

The equation for the circumference of a circle is , where  is the radius of the circle. The radius is half the diameter, or 5 inches.

### Example Question #5 : Circumference

What is the circumference of a circle with radius 3?

Possible Answers:

12.87

9.14

6.45

18.84

Correct answer:

18.84

Explanation:

The equation for circumference of a circle is .

Plug in the given radius value and solve:

### Example Question #6 : Circumference

What is the circumference of a circle with a diameter of 40?

Possible Answers:

Correct answer:

Explanation:

To find the circumference given the diameter we use the equation:

Then we substitute 40 in for our diameter:

### Example Question #7 : Circumference

Find the radius of a circle given that the circumference is .

Possible Answers:

24

5

12

16

Correct answer:

12

Explanation:

The equation of the circumference of a circle is as follows:

Now we substitute in our circumference and solve for radius:

### Example Question #8 : Circumference

What is the circumference of a circle with a radius of 4?

Possible Answers:

Correct answer:

Explanation:

We use the equation for the circumference of a circle:

Now we substitute in our radius of 4:

### Example Question #9 : Circumference

If the area of a circle is , what is its circumference?

Possible Answers:

Cannot be computed from the information provided

Correct answer:

Explanation:

To solve this, you should first figure out your radius.  Remember that the area of a circle is defined as:

For your data, you know that this is:

Solving for , you get:

, or

Now, recall that the circumference of a circle is defined as:

For your data, this is:

### Example Question #10 : Circumference

The area of a sector of a circle with a  degree angle is .  What is the circumference of this circle?

Possible Answers:

Cannot be computed from the information provided

Correct answer:

Explanation:

degree angle represents one fourth of a full circle.  Therefore, the total area of this circle is  or .  Now, recall your area formula:

For your data, this means:

Solving for , you get:

or

Now, the circumference of a circle is defined as:

For your data, this is:

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