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Example Question #772 : Sat Mathematics
A rectangular prism has a length that is twice as long as its width, and a width that is twice as long as its height. If the surface area of the prism is 252 square units, what is the volume, in cubic units, of the prism?
216
27
1728
108
432
216
Let l be the length, w be the width, and h be the height of the prism. We are told that the length is twice the width, and that the width is twice the height. We can set up the following two equations:
l = 2w
w = 2h
Next, we are told that the surface area is equal to 252 square units. Using the formula for the surface area of the rectangular prism, we can write the following equation:
surface area = 2lw + 2lh + 2wh = 252
We now have three equations and three unknowns. In order to solve for one of the variables, let's try to write w and l in terms of h. We know that w = 2h. Because l = 2w, we can write l as follows:
l = 2w = 2(2h) = 4h
Now, let's substitute w = 2h and l = 4h into the equation we wrote for surface area.
2(4h)(2h) + 2(4h)(h) + 2(2h)(h) = 252
Simplify each term.
16h2 + 8h2 + 4h2 = 252
Combine h2 terms.
28h2 = 252
Divide both sides by 28.
h2 = 9
Take the square root of both sides.
h = 3.
This means that h = 3. Because w = 2h, the width must be 6. And because l = 2w, the length must be 12.
Because we now know the length, width, and height, we can find the volume of the prism, which is what the question ultimately requires us to find.
volume of a prism = l • w • h
volume = 12(6)(3)
= 216 cubic units
The answer is 216.
Example Question #5 : Finding Volume Of A Rectangular Prism
The dimensions of Treasure Chest A are 39” x 18”. The dimensions of Treasure Chest B are 16” x 45”. Both are 11” high. Which of the following statements is correct?
Treasure Chest A has the same surface area as Treasure Chest B.
There is insufficient data to make a comparison between Treasure Chest A and Treasure Chest B.
Treasure Chest B can hold more treasure.
Treasure Chest A can hold more treasure.
Treasure Chest A and B can hold the same amount of treasure.
Treasure Chest B can hold more treasure.
The volume of B is 7920 in3. The volume of A is 7722 in3. Treasure Chest B can hold more treasure.
Example Question #2 : How To Find The Volume Of A Prism
Carlos has a pool in the shape of a rectangular prism. He fills the pool with a hose that ejects water at a rate of p gallons per minute. The bottom of the pool is A meters wide and B meters long. If there are k gallons in a cubic meter, then which of the following expressions will be equal to the amount of time it takes, in hours, for the water in the pool to reach a height of c meters?
ABcp/(60k)
ABc/(pk)
ABck/(60p)
60ABc/(pk)
60ABcp/k
ABck/(60p)
Example Question #1 : Cylinders
The volume of a cylinder is 36π. If the cylinder’s height is 4, what is the cylinder’s diameter?
12
9
6
4
3
6
Volume of a cylinder? V = πr2h. Rewritten as a diameter equation, this is:
V = π(d/2)2h = πd2h/4
Sub in h and V: 36p = πd2(4)/4 so 36p = πd2
Thus d = 6
Example Question #2 : Cylinders
A cylinder has a height of 5 inches and a radius of 3 inches. Find the lateral area of the cylinder.
30π
8π
15π
24π
45π
30π
LA = 2π(r)(h) = 2π(3)(5) = 30π
Example Question #3 : Cylinders
A cylinder has a volume of 20. If the radius doubles, what is the new volume?
100
80
20
60
40
80
The equation for the volume of the cylinder is πr2h. When the radius doubles (r becomes 2r) you get π(2r)2h = 4πr2h. So when the radius doubles, the volume quadruples, giving a new volume of 80.
Example Question #171 : Geometry
A cylinder has a height that is three times as long as its radius. If the lateral surface area of the cylinder is 54π square units, then what is its volume in cubic units?
9π
27π
81π
243π
54π
81π
Let us call r the radius and h the height of the cylinder. We are told that the height is three times the radius, which we can represent as h = 3r.
We are also told that the lateral surface area is equal to 54π. The lateral surface area is the surface area that does not include the bases. The formula for the lateral surface area is equal to the circumference of the cylinder times its height, or 2πrh. We set this equal to 54π,
2πrh = 54π
Now we substitute 3r in for h.
2πr(3r) = 54π
6πr2 = 54π
Divide by 6π
r2 = 9.
Take the square root.
r = 3.
h = 3r = 3(3) = 9.
Now that we have the radius and the height of the cylinder, we can find its volume, which is given by πr2h.
V = πr2h
V = π(3)2(9) = 81π
The answer is 81π.
Example Question #2 : Cylinders
What is the volume of a hollow cylinder whose inner radius is 2 cm and outer radius is 4 cm, with a height of 5 cm?
50π cm3
20π cm3
80π cm3
100π cm3
60π cm3
60π cm3
The volume is found by subtracting the inner cylinder from the outer cylinder as given by V = πrout2 h – πrin2 h. The area of the cylinder using the outer radius is 80π cm3, and resulting hole is given by the volume from the inner radius, 20π cm3. The difference between the two gives the volume of the resulting hollow cylinder, 60π cm3.
Example Question #1 : How To Find The Volume Of A Cylinder
What is the volume of a right cylinder with a circumference of 25π in and a height of 41.3 in?
3831.34π in3
25812.5π in3
6453.125π in3
4813.33π in3
1032.5π in3
6453.125π in3
The formula for the volume of a right cylinder is: V = A * h, where A is the area of the base, or πr2. Therefore, the total formula for the volume of the cylinder is: V = πr2h.
First, we must solve for r by using the formula for a circumference (c = 2πr): 25π = 2πr; r = 12.5.
Based on this, we know that the volume of our cylinder must be: π*12.52*41.3 = 6453.125π in3
Example Question #6 : Cylinders
An 8-inch cube has a cylinder drilled out of it. The cylinder has a radius of 2.5 inches. To the nearest hundredth, approximately what is the remaining volume of the cube?
354.92 in3
391.33 in3
462 in3
157.08 in3
203.34 in3
354.92 in3
We must calculate our two volumes and subtract them. The volume of the cube is very simple: 8 * 8 * 8, or 512 in3.
The volume of the cylinder is calculated by multiplying the area of its base by its height. The height of the cylinder is 8 inches (the height of the cube through which it is being drilled). Therefore, its volume is πr2h = π * 2.52 * 8 = 50π in3
The volume remaining in the cube after the drilling is: 512 – 50π, or approximately 512 – 157.0795 = 354.9205, or 354.92 in3.
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