ISEE Middle Level Math : How to find the missing part of a list

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

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Example Questions

Example Question #1 : Sets

What are the next two numbers of this sequence?

\(\displaystyle 10, 14, 19, 23, 28, 32, 37...\)

Possible Answers:

\(\displaystyle 41, 47\)

\(\displaystyle 41, 46\)

\(\displaystyle 41, 45\)

\(\displaystyle 42, 47\)

\(\displaystyle 42, 46\)

Correct answer:

\(\displaystyle 41, 46\)

Explanation:

The sequence is formed by alternately adding \(\displaystyle 4\) and adding \(\displaystyle 5\) to each term to get the next term.

\(\displaystyle \begin{matrix} 10 + 4 = 14\\ 14 + 5 = 19\\ 19+4= 23\\ 23+5=28\\ 28+4 = 32\\ 32+5=37\\ 37+4=41\\ 41+5=46 \end{matrix}\)

\(\displaystyle 41\) and \(\displaystyle 46\) are the next two numbers.

Example Question #1 : Sets

Define two sets as follows:

\(\displaystyle A = \left \{ a, c, d, e, f, h, j, l, p \right \}\)

\(\displaystyle B = \left \{ c, e, f, h, j, o, p, s, z \right \}\)

Which of the following is a subset of \(\displaystyle A \cap B\) ?

Possible Answers:

\(\displaystyle \left \{ c,e,f,h,j,p\right \}\)

\(\displaystyle \left \{ h,c,e,p\right \}\)

Each of the sets listed is a subset of \(\displaystyle A \cap B\).

\(\displaystyle \left \{ f\right \}\)

\(\displaystyle \varnothing\)

Correct answer:

Each of the sets listed is a subset of \(\displaystyle A \cap B\).

Explanation:

We demonstrate that all of the choices are subsets of \(\displaystyle A \cap B\).

\(\displaystyle A \cap B\) is the intersection of \(\displaystyle A\) and \(\displaystyle B\) - that is, the set of all elements of both sets. Therefore, 

\(\displaystyle A \cap B = \left \{ c,e,f,h,j,p\right \}\)

\(\displaystyle \left \{ c,e,f,h,j,p\right \}\) itself is one of the choices; it is a subset of itself. The empty set \(\displaystyle \varnothing\) is a subset of every set. The other two sets listed comprise only elements from \(\displaystyle A \cap B\), making them subsets of \(\displaystyle A \cap B\).

Example Question #1 : How To Find The Missing Part Of A List

Let the universal set \(\displaystyle U\) be the set of all positive integers. Also, define two sets as follows:

\(\displaystyle A = \left \{ 8, 16, 24, 32, 40, 48, 56...\right \}\)

\(\displaystyle B = \left \{ 1, 4, 9, 16, 25, 36, 49,...\right \}\)

Which of the following is an element of the set \(\displaystyle A \cap B\) ?

Possible Answers:

\(\displaystyle 256\)

\(\displaystyle 352\)

\(\displaystyle 484\)

\(\displaystyle 336\)

\(\displaystyle 900\)

Correct answer:

\(\displaystyle 256\)

Explanation:

We are looking for an element that is in the intersection of \(\displaystyle A\) and \(\displaystyle B\) - in other words, we are looking for an element that appears in both sets.

\(\displaystyle A\) is the set of all multiples of 8. We can eliminate two choices as not being in \(\displaystyle A\) by demonstrating that dividing each by 8 yields a remainder:

\(\displaystyle 484 \div 8 = 60 \textrm{ R }4\)

\(\displaystyle 900 \div 8 = 112 \textrm{ R }4\)

\(\displaystyle B\) is the set of all perfect square integers.  We can eliminate two additional choices as not being perfect squares by showing that each is between two consecutive perfect squares:

\(\displaystyle 18^{2}= 324< 352 < 361 = 19^{2}\)

\(\displaystyle 18^{2}= 324< 336 < 361 = 19^{2}\)

This eliminates 352 and 336. However, 

\(\displaystyle 256 = 16 ^{2}\).

It is also a multiple of 8:

\(\displaystyle 256 \div 8 = 32\)

Therefore, \(\displaystyle 256 \in A \cap B\).

Example Question #2 : How To Find The Missing Part Of A List

Define two sets as follows:

\(\displaystyle A = \left \{ 8, 16, 24, 32, 40, 48, 56...\right \}\)

\(\displaystyle B = \left \{ 1, 4, 9, 16, 25, 36, 49,...\right \}\)

Which of the following is not an element of the set \(\displaystyle A \cup B\) ?

Possible Answers:

\(\displaystyle 441\)

\(\displaystyle 420\)

\(\displaystyle 225\)

\(\displaystyle 272\)

\(\displaystyle 344\)

Correct answer:

\(\displaystyle 420\)

Explanation:

\(\displaystyle A \cup B\) is the union of \(\displaystyle A\) and \(\displaystyle B\), the set of all elements that appear in either set. Therefore, we are looking to eliminate the elements in \(\displaystyle A\) and those in \(\displaystyle B\) to find the element in neither set.

\(\displaystyle A\) is the set of all multiples of 8. We can eliminate two choices as mulitples of 8:

\(\displaystyle 272 \div 8 = 34\), so \(\displaystyle 272 \in A\)

\(\displaystyle 344\div 8 = 43\), so \(\displaystyle 344 \in A\)

 

\(\displaystyle B\) is the set of all perfect square integers. We can eliminate two additional choices as perfect squares:

\(\displaystyle \sqrt{225}=15\), so \(\displaystyle 225 \in B\)

\(\displaystyle \sqrt{441}=21\), so \(\displaystyle 441 \in B\)

 

All four of the above are therefore elements of \(\displaystyle A \cup B\).

 

420, however is in neither set:

\(\displaystyle 420 \div 8 = 52 \textrm{ R } 4\), so \(\displaystyle 420 \notin A\)

and 

\(\displaystyle 20 = \sqrt{400}< \sqrt{420} < \sqrt{441} = 21\), so \(\displaystyle 420 \notin B\)

Therefore,  \(\displaystyle 420 \notin A \cup B\), making this the correct choice.

Example Question #1 : How To Find The Missing Part Of A List

Seven students are running for student council; each member of the student body will vote for three. Derreck does not want to vote for Anne, whom he does not like. How many ways can he cast a ballot so as not to include Anne among his choices?

Possible Answers:

\(\displaystyle 40\)

\(\displaystyle 120\)

\(\displaystyle 60\)

\(\displaystyle 20\)

Correct answer:

\(\displaystyle 20\)

Explanation:

Derreck is choosing three students from a field of six (seven minus Anne) without respect to order, making this a combination. He has \(\displaystyle C(6,3)\) ways to choose. This is:

\(\displaystyle C(6,3)= \frac{6!}{(6-3)! \; 3! } = \frac{6!}{3! \; 3! } = \frac{720 }{6 \times 6}= 20\) 

Derreck has 20 ways to fill the ballot.

Example Question #1 : Sets

Ten students are running for Senior Class President. Each member of the student body will choose four candidates, and mark them 1-4 in order of preference. 

How many ways are there to fill out the ballot?

Possible Answers:

\(\displaystyle 151,200\)

\(\displaystyle 10,000\)

\(\displaystyle 210\)

\(\displaystyle 5,040\)

Correct answer:

\(\displaystyle 5,040\)

Explanation:

Four candidates are being selected from ten, with order being important; this means that we are looking for the number of permutations of four chosen from a set of ten. This is

\(\displaystyle P (10,4) = \frac{10!}{(10-4)!}= \frac{10!}{6!} = 10 \times 9 \times 8 \times 7 = 5,040\)

There are 5,040 ways to complete the ballot.

Example Question #2 : Sets

The junior class elections have four students running for President, five running for Vice-President, four running for Secretary-Treasurer, and seven running for Student Council Representative. How many ways can a student fill out a ballot?

Possible Answers:

\(\displaystyle 450\)

\(\displaystyle 630\)

\(\displaystyle 560\)

\(\displaystyle 500\)

Correct answer:

\(\displaystyle 560\)

Explanation:

These are four independent events, so by the multiplication principle, the ballot can be filled out \(\displaystyle 4 \times 5 \times 4\times 7 = 560\) ways.

Example Question #3 : Sets

The sophomore class elections have six students running for President, five running for Vice-President, and six running for Secretary-Treasurer. How many ways can a student fill out a ballot if he is allowed to select one name per office?

Possible Answers:

\(\displaystyle 120\)

\(\displaystyle 90\)

\(\displaystyle 240\)

\(\displaystyle 180\)

Correct answer:

\(\displaystyle 180\)

Explanation:

These are three independent events, so by the multiplication principle, the ballot can be filled out \(\displaystyle 6 \times 5 \times 6 = 180\) ways.

Example Question #4 : Sets

Ten students are running for Senior Class President. Each member of the student body will choose five candidates, and mark them 1-5 in order of preference. 

Roy wants Mike to win. How many ways can Roy fill out the ballot so that Mike is his first choice?

Possible Answers:

\(\displaystyle 3,024\)

\(\displaystyle 210\)

\(\displaystyle 5,040\)

\(\displaystyle 252\)

Correct answer:

\(\displaystyle 3,024\)

Explanation:

Since Mike is already chosen, Roy is in essence choosing four candidates from nine, with order being important. This is a permutation of four elements out of nine. The number of these is

\(\displaystyle P (9,4) = \frac{9!}{(9-4)!} = \frac{9!}{5!} = 9 \times 8 \times 7 \times 6 = 3,024\)

Roy can fill out the ballot 3,024 times and have Mike be his first choice.

Example Question #1 : How To Find The Missing Part Of A List

Find the missing part of the list:

\(\displaystyle \small \small 1, 3, 9, 27,...,243,729\)

Possible Answers:

\(\displaystyle 54\)

\(\displaystyle \small 36\)

\(\displaystyle \small 135\)

\(\displaystyle \small 81\)

\(\displaystyle \small 45\)

Correct answer:

\(\displaystyle \small 81\)

Explanation:

To find the next number in the list, multiply the previous number by \(\displaystyle \small 3\).

\(\displaystyle 27\times3=81\)

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