ISEE Middle Level Quantitative : How to add variables

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

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Example Questions

Example Question #11 : Isee Middle Level (Grades 7 8) Quantitative Reasoning

Define an operation on the real numbers as follows:

For all real values of \(\displaystyle a\) and \(\displaystyle b\),

\(\displaystyle a \bigstar b = ab-a+b\)

\(\displaystyle N\) is a positive number. Which is the greater quantity?

(a) \(\displaystyle N \bigstar N\)

(b) \(\displaystyle (-N) \bigstar (-N)\)

Possible Answers:

(a) and (b) are equal

(a) is the greater quantity

It is impossible to determine which is greater from the information given

(b) is the greater quantity

Correct answer:

(a) and (b) are equal

Explanation:

\(\displaystyle a \bigstar b = ab-a+b\)

so

\(\displaystyle N \bigstar N =N \cdot N -N+N = N^{2}-N+N = N^{2}\)

and 

\(\displaystyle (-N) \bigstar (-N) = (-N) \cdot (-N) - (-N) + (-N)\)

\(\displaystyle = (-1) \cdot N \cdot (-1) \cdot N+N - N\)

\(\displaystyle = N^{2}+N-N\)

\(\displaystyle = N^{2}\)

 

The two are equal regardless of the value of \(\displaystyle N\).

Example Question #11 : Variables

\(\displaystyle x\) and \(\displaystyle y\) are both negative numbers. Which is the greater quantity?

(a) \(\displaystyle (x \div 5) + 0.1 y\)

(b) \(\displaystyle 0.2 x + (y \div 10)\)

Possible Answers:

(a) and (b) are equal

(b) is the greater quantity

It is impossible to determine which is greater from the information given

(a) is the greater quantity

Correct answer:

(a) and (b) are equal

Explanation:

The two quantities are equal regardless of the values of \(\displaystyle x\) and \(\displaystyle y\). To see this, we note that

\(\displaystyle 0.2 x = \frac{2}{10} x = \frac{1}{5} x = \frac{x}{5} = x \div 5\)

and

\(\displaystyle 0.1y = \frac{1}{10} y = \frac{y}{10} = y \div 10\)

Therefore, by the addition property of equality,

\(\displaystyle 0.2 x + (y \div 10) =( x \div 5 )+ 0.1 y\)

Example Question #13 : Isee Middle Level (Grades 7 8) Quantitative Reasoning

\(\displaystyle 5 < A < 7\)

\(\displaystyle 5 < B < 7\)

\(\displaystyle 5 < C < 7\)

Which is the greater quantity?

(a) \(\displaystyle A + B + C\)

(b) 18

Possible Answers:

(a) is the greater quantity

It is impossible to determine which is greater from the information given

(a) and (b) are equal

(b) is the greater quantity

Correct answer:

It is impossible to determine which is greater from the information given

Explanation:

The information is insufficient, as we see by exploring two cases:

Case 1: \(\displaystyle A = B = C = 5\frac{1}{3}\)

\(\displaystyle A+ B + C = 5\frac{1}{3}+ 5\frac{1}{3} + 5\frac{1}{3} = 16 < 18\)

 

Case 2: \(\displaystyle A = B = C = 6\frac{1}{3}\)

\(\displaystyle A+ B + C = 6\frac{1}{3}+ 6\frac{1}{3} + 6\frac{1}{3} =19 > 18\)

Remember, the three variables need not stand for whole numbers.

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