PSAT Math : PSAT Mathematics

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #1 : How To Find The Volume Of A Cone

What is the volume of a cone with a height of  and base radius of ?

Possible Answers:

Correct answer:

Explanation:

The fomula for the volume of a cone is

  

Because our radius of the base is , we know that .

.

Example Question #541 : Geometry

What is the surface area of a cone with a radius of 4 and a height of 3?

Possible Answers:

25\pi

40\pi

36\pi

16\pi

48\pi

Correct answer:

36\pi

Explanation:

Here we simply need to remember the formula for the surface area of a cone and plug in our values for the radius and height.

\Pi r^{2} + \Pi r\sqrt{r^{2} + h^{2}}= \Pi\ast 4^{2} + \Pi \ast 4\sqrt{4^{2} + 3^{2}} = 16\Pi + 4\Pi \sqrt{25} = 16\Pi + 20\Pi = 36\Pi

Example Question #31 : Solid Geometry

The lateral area is twice as big as the base area of a cone.  If the height of the cone is 9, what is the entire surface area (base area plus lateral area)?

Possible Answers:

90π

54π

27π

81π

Correct answer:

81π

Explanation:

Lateral Area = LA = π(r)(l) where r = radius of the base and l = slant height

LA = 2B

π(r)(l) = 2π(r2)

rl = 2r2

l = 2r

Cone

From the diagram, we can see that r2 + h2 = l2.  Since h = 9 and l = 2r, some substitution yields

r2 + 92 = (2r)2 

r2 + 81 = 4r2 

81 = 3r2 

27 = r2

B = π(r2) = 27π

LA = 2B = 2(27π) = 54π

SA = B + LA = 81π

 

Example Question #1 : How To Find The Surface Area Of A Cone

A right cone has a radius of 4R and a height of 3R. What is the ratio of the total surface area of the cone to the surface area of just the base?

Possible Answers:

Correct answer:

Explanation:

We need to find total surface area of the cone and the area of the base. 

The area of the base of a cone is equal to the area of a circle. The formula for the area of a circle is given below:

, where r is the length of the radius.

In the case of this cone, the radius is equal to 4R, so we must replace r with 4R.

To find the total area of the cone, we need the area of the base and the lateral surface area of the cone. The lateral surface area (LA) of a cone is given by the following formula:

, where r is the radius and l is the slant height. 

We know that r = 4R. What we need now is the slant height, which is the distance from the edge of the base of the cone to the tip. 

In order to find the slant height, we need to construct a right triangle with the legs equal to the height and the radius of the cone. The slant height will be the hypotenuse of this triangle. We can use the Pythagorean Theorem to find an expression for l. According to the Pythagorean Theorem, the sum of the squares of the legs (which are 4R and 3R in this case) is equal to the square of the hypotenuse (which is the slant height). According to the Pythagorean Theorem, we can write the following equation:

Let's go back to the formula for the lateral surface area (LA).

To find the total surface area (TA), we must add the lateral area and the area of the base.

The problem requires us to find the ratio of the total surface area to the area of the base. This means we must find the following ratio:

We can cancel , which leaves us with 36/16.

Simplifying 36/16 gives 9/4.

The answer is 9/4.

Example Question #1 : How To Find The Volume Of A Cube

A cubic box has sides of length x. Another cubic box has sides of length 4x. How many of the boxes with length x could fit in one of the larger boxes with side length 4x?

Possible Answers:

80

4

64

40

16

Correct answer:

64

Explanation:

The volume of a cubic box is given by (side length)3. Thus, the volume of the larger box is (4x)3 = 64x3, while the volume of the smaller box is x3. Divide the volume of the larger box by that of the smaller box, (64x3)/(x3) = 64.

Example Question #24 : Solid Geometry

I have a hollow cube with 3” sides suspended inside a larger cube of 9” sides.  If I fill the larger cube with water and the hollow cube remains empty yet suspended inside, what volume of water was used to fill the larger cube?

Possible Answers:

72 in3

702 in3

73 in3

216 in3

698 in3

Correct answer:

702 in3

Explanation:

Determine the volume of both cubes and then subtract the smaller from the larger.  The large cube volume is 9” * 9” * 9” = 729 in3 and the small cube is 3” * 3” * 3” = 27 in3.  The difference is 702 in3.

Example Question #25 : Solid Geometry

A cube weighs 5 pounds. How much will a different cube of the same material weigh if the sides are 3 times as long?

Possible Answers:

10 pounds

15 pounds

135 pounds

45 pounds

Correct answer:

135 pounds

Explanation:

A cube that has three times as long sides is 3x3x3=27 times bigger than the original. Therefore, the answer is 5x27= 135.

Example Question #214 : Geometry

If the volume of a cube is 50 cubic feet, what is the volume when the sides double in length?

Possible Answers:

500 cu ft

100 cu ft

400 cu ft

300 cu ft

200 cu ft

Correct answer:

400 cu ft

Explanation:

Using S as the side length in the original cube, the original is s*s*s. Doubling one side and tripling the other gives 2s*2s*2s for a new volume formula for 8s*s*s, showing that the new volume is 8x greater than the original.

Example Question #215 : Geometry

A cube has 2 faces painted red and the remaining faces painted green. The total area of the green faces is 36 square inches. What is the volume of the cube in cubic inches?

Possible Answers:

16

27

8

54

64

Correct answer:

27

Explanation:

Cubes have 6 faces. If 2 are red, then 4 must be green. We are told that the total area of the green faces is 36 square inches, so we divide the total area of the green faces by the number of green faces (which is 4) to get the area of each green face: 36/4 = 9 square inches. Since each of the 6 faces of a cube have the same size, we know that each edge of the cube is √9 = 3 inches. Therefore the volume of the cube is 3 in x 3 in x 3 in = 27 cubic inches.

Example Question #1 : How To Find The Volume Of A Cube

If a waterproof box is 50cm in length, 20cm in depth, and 30cm in height, how much water will overflow if 35 liters of water are poured into the box?

Possible Answers:

1 liters

30 lites

5 liters

15 liters

No water will flow out of the box

Correct answer:

5 liters

Explanation:

The volume of the box is 50 * 20 * 30 cm = 30,000 cm3.

1cm3 = 1mL, 30,000 cm3 = 30,000mL = 30 L.

Because the volume of the box is only 30 L, 5 L of the 35 L will not fit into the box.

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