ISEE Upper Level Quantitative : Factors / Multiples

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

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Example Questions

Example Question #1 : Numbers And Operations

3/5 + 4/7 – 1/3 = 

Possible Answers:

88/105

3/37

72/89

7/9

Correct answer:

88/105

Explanation:

We need to find a common denominator to add and subtract these fractions. Let's do the addition first. The lowest common denominator of 5 and 7 is 5 * 7 = 35, so 3/5 + 4/7 = 21/35 + 20/35 = 41/35. 

Now to the subtraction. The lowest common denominator of 35 and 3 is 35 * 3 = 105, so altogether, 3/5 + 4/7 – 1/3 = 41/35 – 1/3 = 123/105 – 35/105 = 88/105. This does not simplify and is therefore the correct answer.

Example Question #1 : Numbers And Operations

25 is the greatest common factor of 175 and which of these numbers?

Possible Answers:

None of the answers are correct.

\(\displaystyle 90\)

\(\displaystyle 150\)

\(\displaystyle 77\)

\(\displaystyle 70\)

Correct answer:

\(\displaystyle 150\)

Explanation:

Of the four numbers given, 25 is only a factor of 150, since all multiples of 25 end in the digits 25, 50, 75, or 00. To determine whether 150 is correct, we inspect the factors of 150 and 175:

Factors of 150: \(\displaystyle \left \{ 1, 2, 3,5,6,10,15,\underline{25},30, 50, 75, 150\right \}\)

Factors of 175: \(\displaystyle \left \{ 1,5,7,\underline{25}, 35,175\right \}\)

Since 25 is the greatest number in both lists, \(\displaystyle GCF (150,175) = 25\).

 

Example Question #3 : How To Find The Greatest Common Factor

\(\displaystyle N\) is an odd prime.

Which is the greater quantity?

(a) \(\displaystyle GCF (N,16)\)

(b) \(\displaystyle GCF (N^{4},2)\)

Possible Answers:

It is impossible to tell from the information given.

(b) is equal.

(a) is greater.

(a) and (b) are equal.

Correct answer:

(a) and (b) are equal.

Explanation:

The greatest common factor of two numbers is the product of the prime factors they share; if they share no prime factors, it is \(\displaystyle 1\).

(a) \(\displaystyle 16 = 2 \times 2\times 2\times2\). Since \(\displaystyle N\) is an odd prime, \(\displaystyle N\) and \(\displaystyle 16\) share no prime factors, and \(\displaystyle GCF (N,16) = 1\).

(b) \(\displaystyle N^{4} = N \times N \times N \times N\), since \(\displaystyle N\) is prime. Since \(\displaystyle 2\) is an even prime, \(\displaystyle N^{4}\) and \(\displaystyle 2\) share no prime factors, and \(\displaystyle GCF (N^{4},2) = 1\).

The quantities are equal since each is equal to \(\displaystyle 1\).

Example Question #1 : Greatest Common Factor

Column A               Column B

The GCF of             The GCF of

45 and 120             38 and 114

Possible Answers:

The quantity in Column B is greater.

There is not enough info to determine the relationship.

The quantities are equal.

The quantity in Column A is greater.

Correct answer:

The quantity in Column B is greater.

Explanation:

There are a couple different ways to find the GCF of a set of numbers. Sometimes it's easiest to make a factor tree for each number. The factors that the pair of numbers have in common are then multiplied to get the GCF. So for 45, the prime factorization ends up being: \(\displaystyle 5\cdot 3\cdot 3\). The prime factorization of 120 is: \(\displaystyle 5\cdot 3\cdot 2\cdot 2\cdot 2\). Since they have a 5 and 3 in common, those are multiplied together to get 15 for the GCF. Repeat the same process for 38 and 114. The prime factorization of 38 is \(\displaystyle 19\cdot 2\). The prime factorization of 114 is \(\displaystyle 19\cdot 3\cdot 2\). Therefore, multiply 19 and 2 to get 38 for their GCF. Column B is greater.

Example Question #2 : Greatest Common Factor

What is the greatest common factor of \(\displaystyle 630\) and \(\displaystyle 660\)?

Possible Answers:

\(\displaystyle 15\)

\(\displaystyle 60\)

\(\displaystyle 13860\)

\(\displaystyle 30\)

\(\displaystyle 13860\)

Correct answer:

\(\displaystyle 30\)

Explanation:

To solve for the greatest common factor, it is necessary to get your numbers into prime factor form. For each of your numbers, this is:

\(\displaystyle 630 = 63 * 10 = 7 * 9 * 2 * 5 = 7 * 3 * 3 * 2 * 5\)

        \(\displaystyle = 2 * 3^2* 5*7\)

\(\displaystyle 660 = 66 * 10 = 6 * 11 * 2 * 5 = 2 * 3 * 11 * 2 *5\)

        \(\displaystyle = 2^2 *3*5*11\) 

Next, for each of your sets of prime factors, you need to choose the exponent for which you have the smallest value; therefore, for your values, you choose:

\(\displaystyle 2\)\(\displaystyle 2\)

\(\displaystyle 3\)\(\displaystyle 3\)

\(\displaystyle 5\)\(\displaystyle 5\)

\(\displaystyle 7\): None

\(\displaystyle 11\):  None

Taking these together, you get:

\(\displaystyle 2*3*5 = 30\)

 

Example Question #1 : How To Find The Greatest Common Factor

What is the greatest common factor of \(\displaystyle 520\) and \(\displaystyle 175\)?

Possible Answers:

\(\displaystyle 40\)

\(\displaystyle 5\)

\(\displaystyle 91\)

\(\displaystyle 1820\)

\(\displaystyle 3640\)

Correct answer:

\(\displaystyle 5\)

Explanation:

To solve for the greatest common factor, it is necessary to get your numbers into prime factor form. For each of your numbers, this is:

\(\displaystyle 520 = 52 * 10 = 13 * 4 * 2 * 5 = 13 * 2 * 2* 2 *5\)

        \(\displaystyle = 2^3 *5*13\)

\(\displaystyle 175 = 5 * 35 = 5 * 5 * 7 = 5^2 * 7\) 

Next, for each of your sets of prime factors, you need to choose the exponent for which you have the smallest value; therefore, for your values, you choose:

\(\displaystyle 2\): None

\(\displaystyle 5\)\(\displaystyle 5\)

\(\displaystyle 7\): None

\(\displaystyle 13\): None

Taking these together, you get: \(\displaystyle 5\)

Example Question #3 : Greatest Common Factor

What is the greatest common factor of \(\displaystyle 208\) and \(\displaystyle 204\)?

Possible Answers:

\(\displaystyle 4\)

\(\displaystyle 10608\)

\(\displaystyle 5304\)

\(\displaystyle 429\)

\(\displaystyle 12\)

Correct answer:

\(\displaystyle 4\)

Explanation:

To solve for the greatest common factor, it is necessary to get your numbers into prime factor form. For each of your numbers, this is:

\(\displaystyle 208 = 4 * 52 = 2 * 2 * 4*13 = 2 * 2 * 2 * 2* 13\)

        \(\displaystyle =2^4 * 13\)

\(\displaystyle 204 = 6 * 34 = 2 * 3 * 2 * 17 = 2^2 * 3 * 17\) 

Next, for each of your sets of prime factors, you need to choose the exponent for which you have the smallest value; therefore, for your values, you choose:

\(\displaystyle 2\)\(\displaystyle 2^2\)

\(\displaystyle 3\): None

\(\displaystyle 13\): None

\(\displaystyle 17\): None

Taking these together, you get: \(\displaystyle 2^2 = 4\)

Example Question #1 : Factors / Multiples

Annette's family has \(\displaystyle 8\frac{3}{5}\) jars of applesauce. In a month, they go through \(\displaystyle 4\frac{2}{3}\) jars of apple sauce. How many jars of applesauce remain?

Possible Answers:

\(\displaystyle 4\frac{14}{15}\)

\(\displaystyle 4\)

\(\displaystyle 3\frac{13}{15}\)

\(\displaystyle 3\frac{14}{15}\)

Correct answer:

\(\displaystyle 3\frac{14}{15}\)

Explanation:

If Annette's family has \(\displaystyle 8\frac{3}{5}\) jars of applesauce, and in a month, they go through \(\displaystyle 4\frac{2}{3}\) jars of apple sauce, that means \(\displaystyle 8\frac{3}{5}-4\frac{2}{3}\) jars of applesauce will be left. 

The first step to determining how much applesauce is left it to convert the fractions into mixed numbers. This gives us:

\(\displaystyle \frac{43}{5}-\frac{14}{3}\)

The next step is to find a common denominator, which would be 15. This gives us:

\(\displaystyle \frac{129}{15}-\frac{70}{15}\)

\(\displaystyle \frac{59}{15}\)

\(\displaystyle 3\frac{14}{15}\)

 

Example Question #2 : Factors / Multiples

\(\displaystyle a\), \(\displaystyle b\), \(\displaystyle c\), \(\displaystyle d\), and \(\displaystyle e\) are five distinct prime integers. Give the greatest common factor of \(\displaystyle abc^{2}d\) and \(\displaystyle a^{2}bde^{2}\).

Possible Answers:

\(\displaystyle a^{2}bd\)

\(\displaystyle abcde\)

\(\displaystyle a^{2}bc^{2}de^{2}\)

\(\displaystyle a^{2}b^{2}c^{2}d^{2}e^{2}\)

\(\displaystyle abd\)

Correct answer:

\(\displaystyle abd\)

Explanation:

If two integers are broken down into their prime factorizations, their greatest common factor is the product of their common prime factors.

Since \(\displaystyle a\), \(\displaystyle b\), \(\displaystyle c\), \(\displaystyle d\), and \(\displaystyle e\) are distinct prime integers, the two expressions can be factored into their prime factorizations as follows - with their common prime factors underlined:

\(\displaystyle abc^{2}d = \underline{a} \cdot \underline{b} \cdot c \cdot c \cdot \underline{d}\)

\(\displaystyle a^{2}bde^{2} = \underline{a} \cdot a \cdot \underline{b} \cdot \underline{d} \cdot e \cdot e\)

The greatest common factor is the product of those three factors, or \(\displaystyle abd\).

Example Question #1 : How To Factor A Number

Which of these numbers is relatively prime with 18?

Possible Answers:

\(\displaystyle 35\)

\(\displaystyle 39\)

\(\displaystyle 34\)

\(\displaystyle 32\)

\(\displaystyle 33\)

Correct answer:

\(\displaystyle 35\)

Explanation:

For two numbers to be relatively prime, they cannot have any factor in common except for 1. The factors of 18 are 1, 2, 3, 6, 9, and 18.

We can eliminate 32 and 34, since each shares with 18 a factor of 2; we can also eliminate 33 and 39, since each shares with 18 a factor of 3. The factors of 35 are 1,  5, 7, and 35; as can be seen by comparing factors, 18 and 35 only have 1 as a factor, making 35 the correct choice.

 

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