GMAT Math : GMAT Quantitative Reasoning

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Example Questions

Example Question #15 : Calculating The Length Of A Radius

If a monster truck's wheels have circumference of , what is the distance from the ground to the center of the wheel?

Possible Answers:

3ft

2.5ft

5ft

10ft

Correct answer:

5ft

Explanation:

If a monster truck's wheels have circumference of , what is the distance from the ground to the center of the wheel?

This question is asking us to find the radius of a circle. the distance from the outside of the circle to the center is the radius. We are given the circumference, so use the following formula:

$Circ=2 \pi r$

Then, plug in what we know and solve for r

$31.4=2 \pi r$

$r=\frac{31.4}{2 \pi}=4.997\approx5ft$

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Example Question #71 : Circles

Two circles in the same plane have the same center. The smaller circle has radius 10; the area of the region between the circles is $50 \pi$ . What is the radius of the larger circle?

Possible Answers:

$5 \sqrt{2}$

$6 \sqrt{5}$

$5 \sqrt{6}$

Correct answer:

$5 \sqrt{6}$

Explanation:

The area of a circle with radius is $A = \pi r^{2}$ .

Let be the radius of the larger circle. Its area is $A_{1} = \pi R^{2}$ . The area of the smaller circle is $A_{2} = \pi \cdot 10^{2} = 100 \pi$ . Since the area of the region between the circles is $50 \pi$ , and is the difference of these areas, we have

$\pi R^{2} - 100 \pi = 50 \pi$

$\pi R^{2} - 100 \pi+ 100 \pi = 50 \pi + 100 \pi$

$\pi R^{2} = 150 \pi$

$R^{2} = 150$

$R = \sqrt{150} = \sqrt {25 } \cdot \sqrt{6} = 5 \sqrt{6}$

The smaller circle has radius $5 \sqrt{6}$ .

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Example Question #1 : Calculating Circumference

A circle on the coordinate plane has equation

$x^{2} + y ^{2} = 40$

Which of the following represents its circumference?

Possible Answers:

$80 \pi$

$16 \pi \sqrt{10}$

$4 \pi \sqrt{10}$

$40 \pi$

$8 \pi \sqrt{10}$

Correct answer:

$4 \pi \sqrt{10}$

Explanation:

The equation of a circle centered at the origin is

$x^{2} + y ^{2} = r ^{2}$

where is the radius of the circle.

In this equation, $r ^{2} = 40$ , so $r = \sqrt{40}$ ; this simplifies to

$r = \sqrt{40} = \sqrt{4} \cdot \sqrt{10} = 2 \sqrt{10}$

The circumference of a circle is $C = 2 \pi r$ , so substitute $r = 2 \sqrt{10}$ :

$C = 2 \pi r = 2 \pi \cdot 2 \sqrt{10} = 4 \pi \sqrt{10}$

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Example Question #1 : Calculating Circumference

A circle on the coordinate plane has equation

$x ^{2} - 8x + y ^{2} - 10x = 7$

What is its circumference?

Possible Answers:

$4 \pi \sqrt{ 3}$

$48 \pi$

$16 \pi \sqrt{ 3}$

$8 \pi \sqrt{ 3}$

$96 \pi$

Correct answer:

$8 \pi \sqrt{ 3}$

Explanation:

The standard form of the area of a circle with radius and center (h,k ) is

$(x-h)^{2} + (y-k)^{2} = r^{2}$

Once we get the equation in standard form, we can find radius , and multiply it by $2 \pi$ to get the circumference.

Complete the squares:

$\left (\frac{-8}{2} \right )^{2} = 16,\left (\frac{-10}{2} \right )^{2} = 25$

so $x ^{2} - 8x + y ^{2} - 10x = 7$ can be rewritten as follows:

$x ^{2} - 8x + 16 + y ^{2} - 10x + 25 = 7 + 16 + 25$

$\left ( x -4 \right )^{2} + \left ( y -5 \right ) ^{2} = 48$

$r ^{2} = 48$ ,

so $r = \sqrt{48} = \sqrt{16} \cdot \sqrt{3} = 4 \sqrt{3}$

And $C = 2\pi \cdot 4 \sqrt{ 3} = 8 \pi \sqrt{ 3}$

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Example Question #311 : Gmat Quantitative Reasoning

On average, Stephanie walks $2\pi$ feet every seconds. If Stephanie walks at her usual pace, how long will it take her to walk around a circular track with a radius of 200 feet, in seconds?

Possible Answers:

$400 \pi$ seconds

400 seconds

800 seconds

$800 \pi$ seconds

None of the other answers are correct.

Correct answer:

800 seconds

Explanation:

The length of the track equals the circumference of the circle.

$C = 2\pi r$

$C=2\pi (200)=400\pi$

$Speed = \frac{distance}{time}$

Therefore, $time = \frac{distance}{speed}$ .

$time = 400\pi\: ft \div \frac{2\pi \: ft}{4\: sec} = 400\pi\: ft \times\frac{4\:sec}{2\pi\: feet}$

$time= 400\pi\: ft \times\frac{2\:sec}{1\pi\: feet} = 800\:seconds$

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Example Question #2 : Calculating Circumference

A circle on the coordinate plane has equation $x^{2} + y ^{2} = 64$ .

What is its circumference?

Possible Answers:

$8 \pi$

$64 \pi$

$16 \pi ^2$

$8 \pi ^2$

$16 \pi$

Correct answer:

$16 \pi$

Explanation:

The equation of a circle centered at the origin is

$x^{2} + y ^{2} = r ^{2}$ ,

where is the radius of the circle.

In the equation given in the question stem, $r ^{2} = 64$ , so $r = \sqrt{64} = 8$ .

The circumference of a circle is $C = 2 \pi r$ , so substitute r = 8 :

$C = 2 \pi r = 2 \pi \cdot 8 = 16 \pi$

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Example Question #2 : Calculating Circumference

Let A, B be concentric circles. Circle has a radius of , and the shortest distance from the edge of circle to the edge of circle is . What is the circumference of circle ?

Possible Answers:

$10\pi$

$6\pi$

$40\pi$

$\pi$

$14\pi$

Correct answer:

$40\pi$

Explanation:

Since A,B are concentric circles, they share a common center, like sections of a bulls-eye target. Since the radius of is less than half the distance from the edge of to the edge of , we must have circle is inside of circle . (It's helpful to draw a picture to see what's going on!)

Now we can find the radius of by adding and , which is 20. And the equation for finding circumfrence is $C =2r\pi$ . Plugging in for gives $40\pi$ .

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Example Question #3 : Calculating Circumference

Consider the Circle :

Circle3

(Figure not drawn to scale.)

Suppose Circle represents a circular pen for Frank's mules. How many meters of fencing does Frank need to build this pen?

Possible Answers:

$45 \pi\:m$

$225 \pi\:m$

$30 \pi\:m$

$60 \pi\:m$

$15 \pi\:m$

Correct answer:

$30 \pi\:m$

Explanation:

We need to figure out the length of fencing needed to surround a circular enclosure, or in other words, the circumference of the circle.

Circumference equation:

$C=2 \pi r$

Where is our radius, which is $15\:m$ in this case. Plug it in and simplify:

$C=2 \pi *15=30 \pi\:m$

And we have our answer!

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Example Question #2 : Calculating Circumference

If the radius of a circle is $5\:cm$ , what is its circumference?

Possible Answers:

$5\pi\:cm$

$25\pi\:cm$

$10\pi\:cm$

$2.5\pi\:cm$

$15\pi\:cm$

Correct answer:

$10\pi\:cm$

Explanation:

Using the formula for the circumference of a circle, we can plug in the given value for the radius and calculate our solution:

$C=2\pi r=2\pi (5\:cm)=10\pi\:cm$

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Example Question #8 : Calculating Circumference

Megan, a civil engineer, is designing a roundabout for the city of Madison. She knows that the distance from the edge of the roundabout to the center must be 25 meters. Help Megan find the circumference of the roundabout.

Possible Answers:

$50\pi m$

$12.5\pi m$

$5\pi m$

$225\pi m$

Correct answer:

$50\pi m$

Explanation:

We are asked to find circumference. In order to do so, look at the following formula:

$C=2\pi r$

Where r is our radius and C is our circumference.

We are indirectly told that our radius is 25 meters, plug it in to get our answer:

$C=2\pi 25m=50 \pi m$

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GMAT Math : GMAT Quantitative Reasoning

Study concepts, example questions & explanations for GMAT Math

All GMAT Math Resources

Example Questions

Example Question #15 : Calculating The Length Of A Radius

Example Question #71 : Circles

Example Question #1 : Calculating Circumference

Example Question #1 : Calculating Circumference

Example Question #311 : Gmat Quantitative Reasoning

Example Question #2 : Calculating Circumference

Example Question #2 : Calculating Circumference

Example Question #3 : Calculating Circumference

Example Question #2 : Calculating Circumference

Example Question #8 : Calculating Circumference

All GMAT Math Resources

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