ACT Math : Solid Geometry

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #1 : Solid Geometry

If we have a regular (the triangles are equilateral) triangular prism of volume \(\displaystyle 32 in^3\) and the side length of the triangle on either face is \(\displaystyle 3 in\), what is the length of the prism? Write your answer in terms of a decimal rounded to the nearest hundredth.

Possible Answers:

\(\displaystyle 3.56\ inches\)

Not enough information to decide.

\(\displaystyle 10.67\ inches\)

\(\displaystyle 12.32\ inches\)

Correct answer:

\(\displaystyle 12.32\ inches\)

Explanation:

The volume of a triangular prism can be simply stated as V = A*L, where A is the area of the triangular face and L is the length. We have the volume already and we need to figure out the area of the triangle from the side length.

The area of a triangle is b*h/2, and we are given the base/side length: 3 in. We also have a formula for the height of an equilateral triangle, \(\displaystyle b\frac{\sqrt{3}}{2}\). Calculating the area of the triangle we get 2.5981 in^2, so now we just have to plug our numbers into the volume formula.

\(\displaystyle V = A\cdot L\Rightarrow 32 in^3 = 2.5981 in^2 \cdot L\)

\(\displaystyle L = \frac {32}{2.5981} in = 12.32 in\)

And that is the final answer.

Example Question #1 : Solid Geometry

Roberto has a swimming pool that is in the shape of a rectangular prism. His swimming pool is \(\displaystyle 25\) meters wide, \(\displaystyle 10\) meters long, and \(\displaystyle 3\) meters deep. He needs to fill up the pool for summer, and his hose fills at a rate of \(\displaystyle 5\) cubic meters per hour. How many hours will it take for Roberto to fill up the swimming pool?

Possible Answers:

\(\displaystyle 75\)

\(\displaystyle 150\)

\(\displaystyle 100\)

\(\displaystyle 50\)

Correct answer:

\(\displaystyle 150\)

Explanation:

First, find the volume of the pool. For a rectangular prism, the formula for the volume is the following:

\(\displaystyle \text{Volume}=\text{length}\times \text{width}\times \text{height}\)

For the swimming pool,

\(\displaystyle \text{Volume}=10 \times 25\times 3=750\) cubic meters

Now, because the hose only fills up \(\displaystyle 5\) cubic meters per hour, divide the total volume by \(\displaystyle 5\) to find how long it will take for the pool to fill.

\(\displaystyle 750\div5=150\)

It will take the pool \(\displaystyle 150\) hours to fill by hose.

Example Question #1 : Solid Geometry

Matt baked a rectangular cake for his mom's birthday. The cake was \(\displaystyle 15\) inches long, \(\displaystyle 12\) inches wide, and \(\displaystyle 4\) inches high. If he cuts the cake into pieces that are \(\displaystyle 3\) inches long, \(\displaystyle 2\) inches wide, and \(\displaystyle 4\) inches high, how many pieces of cake can he cut?

Possible Answers:

\(\displaystyle 30\)

\(\displaystyle 28\)

\(\displaystyle 24\)

\(\displaystyle 40\)

Correct answer:

\(\displaystyle 30\)

Explanation:

First, find the volume of the cake. For a rectangular prism,

\(\displaystyle \text{Volume}=\text{length}\times\text{width}\times\text{height}\)

\(\displaystyle \text{Volume}=15\times12\times4=720\)

Next, find the volume of each individual slice.

\(\displaystyle \text{Volume}=3\times2\times4=24\)

Now, divide the volume of the entire cake by the volume of the slice to get how many pieces of cake Matt can cut.

\(\displaystyle 720\div24=30\)

Example Question #1 : Solid Geometry

A cube has a surface area of \(\displaystyle 150\textup{m}^2\). What is its volume?

Possible Answers:

\(\displaystyle 225\textup{m}^3\)

\(\displaystyle 1000\textup{m}^3\)

\(\displaystyle 300\textup{m}^3\)

\(\displaystyle 125\textup{m}^3\)

Correct answer:

\(\displaystyle 125\textup{m}^3\)

Explanation:

First, find the side lengths of the cube.

Recall that the surface area of the cube is given by the following equation:

\(\displaystyle \text{Surface Area}=6s^2\), where \(\displaystyle s\) is the length of a side.

Plugging in the surface area given by the equation, we can then find the side length of the cube.

\(\displaystyle 150=6s^2\)

\(\displaystyle s^2=25\)

\(\displaystyle s=5\)

Now, recall that the volume of a cube is given by the following equation:

\(\displaystyle \text{Volume}=s^3\)

\(\displaystyle \text{Volume}=5^3=125\)

Example Question #1 : Solid Geometry

The volume of the right triangular prism is \(\displaystyle 120\textup{in}^{3}\). Find the value of \(\displaystyle x\).

1

Possible Answers:

\(\displaystyle 3\)

\(\displaystyle 4\)

\(\displaystyle 2\)

\(\displaystyle 6\)

Correct answer:

\(\displaystyle 4\)

Explanation:

The volume of a right triangular prism is given by the following equation:

\(\displaystyle \text{Volume}=\text{Area of the Base}\times \text{height}\)

Now, for the given question, the height is \(\displaystyle 12\).

\(\displaystyle 120=\text{Area of the base}\times 12\)

\(\displaystyle \text{Area of the base}=10\)

Since the area of the base is a right triangle, we can plug in the given values to find \(\displaystyle x\).

\(\displaystyle \text{Area of Triangle}=\frac{\text{base}\times\text{height}}{2}\)

\(\displaystyle 10=\frac{5x}{2}\)

\(\displaystyle 5x=20\)

\(\displaystyle x=4\)

Example Question #2 : Solid Geometry

The tent shown below is in the shape of a triangular prism. What is the volume of this tent in cubic feet?

2

Possible Answers:

\(\displaystyle 360\textup{ft}^{3}\)

\(\displaystyle 120\textup{ft}^{3}\)

\(\displaystyle 80\textup{ft}^{3}\)

\(\displaystyle 240\textup{ft}^{3}\)

Correct answer:

\(\displaystyle 120\textup{ft}^{3}\)

Explanation:

The volume of a right triangular prism is given by the following equation:

\(\displaystyle \text{Volume}=\text{Area of the Base}\times \text{height}\)

\(\displaystyle \text{Area of Base}=\frac{5\times6}{2}=15\)

\(\displaystyle \text{Volume}=15\times 8=120\)

Example Question #1 : Other Polyhedrons

The height of a box is twice its width and half its length. If the volume of the box is \(\displaystyle 64\textup{yd}^{3}\), what is the length of the box?

Possible Answers:

\(\displaystyle 4\textup{ yards}\)

\(\displaystyle 8\textup{ yards}\)

\(\displaystyle 2\textup{ yards}\)

\(\displaystyle 6\textup{ yards}\)

Correct answer:

\(\displaystyle 8\textup{ yards}\)

Explanation:

For a rectangular prism, the formula for the volume is the following:

\(\displaystyle \text{Volume}=\text{length}\times \text{width}\times \text{height}\)

Now, we know that the height is twice its width. We can rewrite that as:

\(\displaystyle h=2w\)

\(\displaystyle w=\frac{h}{2}\)

We also know that the height is half its length. That can be written as:

\(\displaystyle h=\frac{1}{2}l\)

\(\displaystyle l=2h\)

Now, we can plug in the values of the length, width, and height in terms of height to find the height.

\(\displaystyle 64=(2h)(\frac{h}{2})(h)\)

\(\displaystyle 64=h^3\)

\(\displaystyle h=4\)

The question wants to find the length of the box. Plug in the value of the height in the earlier equation we wrote earlier to represent the relatioinship between the height and the length.

\(\displaystyle l=2h=2(4)=8\)

Example Question #1 : Other Polyhedrons

In cubic inches, find the volume of a tetrahedron that has a surface area of \(\displaystyle 100\sqrt3\textup{ in}^{2}\).

Possible Answers:

\(\displaystyle \frac{250\sqrt2}{3}\)

\(\displaystyle 1000\)

\(\displaystyle 500\sqrt3\)

\(\displaystyle \frac{500\sqrt2}{3}\)

Correct answer:

\(\displaystyle \frac{250\sqrt2}{3}\)

Explanation:

First, we will need to find the length of a side of the tetrahedron.

We can use the surface area to find the lengh of a side. Recall that the formula to find the surface of a tetrahedron:

\(\displaystyle \text{Surface Area}=\sqrt3 s^2\), where \(\displaystyle s\) is the side length.

\(\displaystyle 100\sqrt3=\sqrt3s^2\)

\(\displaystyle s^2=100\)

\(\displaystyle s=10\)

Now, recall the formula to find the volume of a tetrahedron:

\(\displaystyle \text{Volume}=\frac{\sqrt2}{12}s^3\)

\(\displaystyle \text{Volume}=\frac{\sqrt2}{12}10^3=\frac{1000\sqrt2}{12}=\frac{250\sqrt2}{3}\)

Example Question #1 : Solid Geometry

Susan bought a chocolate bar that came in a container shaped like a triangular prism shown below. If the container is completely filled with chocolate, in cubic inches, what volume of chocoate did Susan buy?

3

Possible Answers:

\(\displaystyle 78\textup{in}^{3}\)

\(\displaystyle 96\textup{in}^{3}\)

\(\displaystyle 56\textup{in}^{3}\)

\(\displaystyle 112\textup{in}^{3}\)

Correct answer:

\(\displaystyle 56\textup{in}^{3}\)

Explanation:

The volume of a right triangular prism is given by the following equation:

\(\displaystyle \text{Volume}=\text{Area of the Base}\times \text{height}\)

\(\displaystyle \text{Area of Base}=\frac{4\times2}{2}=4\)

\(\displaystyle \text{Volume}=4\times 14=56\)

Example Question #3 : Solid Geometry

Troy's company manufactures dice that are shaped like cubes and have side lengths of \(\displaystyle 2\textup{cm}\). If the plastic needed to make the dice costs \(\displaystyle \$0.10\) per cubic centimeter, how much does it cost Troy to make one die?

Possible Answers:

\(\displaystyle \$0.80\)

\(\displaystyle \$1.60\)

\(\displaystyle \$0.60\)

\(\displaystyle \$0.40\)

Correct answer:

\(\displaystyle \$0.80\)

Explanation:

First, find the volume of the die. For a cube, the volume has the following formula:

\(\displaystyle \text{Volume}=\text{side length}^3\)

\(\displaystyle \text{Volume}=2^3=8\)

Because it costs \(\displaystyle \$0.10\) for each cubic centimeter, you will need to multiply this number by \(\displaystyle 8\) to get the cost of each die.

\(\displaystyle 8(\$0.10)=\$0.80\)

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