ISEE Upper Level Quantitative : How to find the length of an edge

Study concepts, example questions & explanations for ISEE Upper Level Quantitative

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Example Questions

Example Question #1 : Prisms

A cube has sidelength one and one-half feet; a rectangular prism of equal volume has length 27 inches and height 9 inches. Give the width of the prism in inches.

Possible Answers:

\(\displaystyle 27 \textrm{ in}\)

\(\displaystyle 18 \textrm{ in}\)

\(\displaystyle 30\textrm{ in}\)

\(\displaystyle 36\textrm{ in}\)

\(\displaystyle 24 \textrm{ in}\)

Correct answer:

\(\displaystyle 24 \textrm{ in}\)

Explanation:

One and one half feet is equal to eighteen inches, so the volume of the cube, in cubic inches, is the cube of this, or

\(\displaystyle V = 18^{3} = 5,832\) cubic inches.

The volume of a rectangular prism is

\(\displaystyle V = LHW\)

Since its volume is the same as that of the cube, and its length and height are 27 and 9 inches, respectively,  we can rewrite this as

\(\displaystyle 5,832= 27 \cdot 9 \cdot W\)

\(\displaystyle 5,832= 243 \cdot W\)

\(\displaystyle W = 5,832 \div 243 = 24\)

The width is 24 inches.

Example Question #2 : Solid Geometry

A cube has sidelength one and one-half feet; a rectangular prism of equal surface area has length 27 inches and height 9 inches. Give the width of the prism in inches.

Possible Answers:

\(\displaystyle 24\textrm{ in}\)

\(\displaystyle 27\frac{1}{2} \textrm{ in}\)

\(\displaystyle 20 \frac{1}{4} \textrm{ in}\)

\(\displaystyle 22 \frac{1}{2} \textrm{ in}\)

\(\displaystyle 18\textrm{ in}\)

Correct answer:

\(\displaystyle 20 \frac{1}{4} \textrm{ in}\)

Explanation:

One and one half feet is equal to eighteen inches, so the surface area of the cube, in square inches, is six times the square of this, or

\(\displaystyle 6 \cdot 18^{2} = 6 \cdot 324 = 1,944\) square inches.

The surface area of a rectangular prism is determined by the formula

\(\displaystyle A = 2LW + 2LH + 2 WH\).

So, with substitutiton, we can find the width:

\(\displaystyle 1,944= 2 \cdot 27 \cdot W + 2\cdot 27 \cdot 9 + 2 \cdot W \cdot 9\)

\(\displaystyle 1,944= 54W +486+ 18 W\)

\(\displaystyle 72W +486 = 1,944\)

\(\displaystyle 72W +486 - 486 = 1,944 - 486\)

\(\displaystyle 72W = 1,458\)

\(\displaystyle 72W\div 72 = 1,458 \div 72\)

\(\displaystyle W = 20 \frac{1}{4}\) inches

 

Example Question #2 : Prisms

A rectangular prism has volume one cubic foot; its length and width are, respectively, 9 inches and \(\displaystyle t\) inches. Which of the following represents the height of the prism in inches?

Possible Answers:

\(\displaystyle \frac{192}{t}\)

\(\displaystyle 16t\)

\(\displaystyle \frac{96}{t}\)

\(\displaystyle 96t\)

\(\displaystyle \frac{16}{t}\)

Correct answer:

\(\displaystyle \frac{192}{t}\)

Explanation:

The volume of a rectangular prism is the product of its length, its width, and its height. The prism's volume of one cubic foot is equal to \(\displaystyle 12^{3} = 1,728\) cubic inches.

Therefore, \(\displaystyle LWH = V\) can be rewritten as \(\displaystyle 9 \cdot t \cdot H = 1,728\).

We can solve for \(\displaystyle H\) as follows:

\(\displaystyle \frac{9 t \cdot H }{9t}= \frac{1,728 }{9t}\)

\(\displaystyle H = \frac{1,728 \div 9 }{9t \div 9 }\)

\(\displaystyle H = \frac{192}{t}\)

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