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# Counting Principles

Permutations & Combinations, Factorial Notation, Product Principle, Sum Principle…

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##### Question 1

calculator

easy

[Maximum mark: 4]

Find the number of ways in which ten different books can be given to Ethan, Henry, Joshua and Lucy, if Ethan is to receive $4$ books, Henry is to receive $3$ books, Joshua is to receive $2$ books and Lucy is to receive $1$ book.

easy

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##### Question 2

calculator

easy

[Maximum mark: 4]

Find the number of ways in which twelve different baseball cards can be given to Emily, Harry, John and Olivia, if Emily is to receive $5$ cards, Harry is to receive $3$ cards, John is to receive $3$ cards and Olivia is to receive $1$ card.

easy

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##### Question 3

calculator

easy

[Maximum mark: 6]

A school basketball team of $5$ students is selected from $8$ boys and $4$ girls.

1. Determine how many possible teams can be chosen. 

2. Determine how many teams can be formed consisting of $3$ boys and $2$ girls? 

3. Determine how many teams can be formed consisting of at most $3$ girls? 

easy

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##### Question 4

calculator

easy

[Maximum mark: 6]

A police department has $4$ male and $7$ female officers. A special group of $5$ officers is to be assembled for an undercover operation.

1. Determine how many possible groups can be chosen. 

2. Determine how many groups can be formed consisting of $2$ males and $3$ $\text{females.}$

3. Determine how many groups can be formed consisting of at least one male. 

easy

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##### Question 5

calculator

easy

[Maximum mark: 4]

Mark has created an online test for his Physics students to practice in class. There are $12$ questions to be attempted in random order. Two of the questions require the use of a calculator.

Find the number of different ways in which a student can attempt the test and encounter the two calculator required questions consecutively.

easy

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##### Question 6

calculator

medium

[Maximum mark: 4]

Peter needs to decide the order in which to schedule $14$ exams for his school. Two of these exams are Chemistry ($1$ SL and $1$ HL).

Find the number of different ways Peter can schedule the $14$ exams given that the two Chemistry subjects must not be consecutive.

medium

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##### Question 7

calculator

medium

[Maximum mark: 6]

Ten students are to be arranged in a new chemistry lab. The chemistry lab is set out in two rows of five desks as shown in the following diagram. 1. Find the number of ways the ten students may be arranged in the lab. 

Two of the students, Hugo and Leo, were noticed to talk to each other during previous lab sessions.

1. Find the number of ways the students may be arranged if Hugo and Leo must sit so that one is directly behind the other. For example, Dest $1$ and Desk $6$. 

2. Find the number of ways the students may be arranged if Hugo and Leo must not sit next to each other in the same row. 

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##### Question 8

calculator

medium

[Maximum mark: 6]

Julie works at a book store and has nine books to display on the main shelf of the store. Four of the books are non-fiction and five are fiction. Each book is different. Determine the number of possible ways Julie can line up the nine books on the main shelf, given that

1. the non-fiction books should stand together; 

2. the non-fiction books should stand together on either end; 

3. the non-fiction books should stand together and do not stand on either end. 

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##### Question 9

calculator

medium

[Maximum mark: 6]

Six women and two men wait in a grocery store checkout line. Determine the number of possible ways these customers can stand in line, given that

1. the men do not stand together; 

2. the men do not stand on either end; 

3. the men do not stand together and do not stand on either end. 

medium

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##### Question 10

calculator

medium

[Maximum mark: 5]

Sophia and Zoe compete in a freestyle swimming race where there are no tied finishes and there is a total of $10$ competitors.

Find the total number of possible ways in which the ten swimmers can finish if Zoe finishes

1. in the position immediately after Sophia;

2. in any position after Sophia.

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##### Question 11

calculator

medium

[Maximum mark: 5]

A music company announces that they are going to release a list of their top 10 artists. If 5 of them are pop artists, 3 of them are rock artists and 2 are hip-hop artists, find the total number of possible ways in which

1. the rock artists are listed consecutively;

2. the rock artists and the hip-hop artists are both listed consecutively, with the 3 rock artists listed immediately below the 2 hip-hop artists.

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##### Question 12

calculator

hard

[Maximum mark: 6]

There are $11$ players on a football team who are asked to line up in one straight line for a team photo. Three of the team members named Adam, Brad and Chris refuse to stand next to each other. There is no restriction on the order in which the other team members position themselves.

Find the number of different orders in which the $11$ team members can be positioned for the photo.

hard

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##### Question 13

no calculator

hard

[Maximum mark: 7]

There are six office cubicles arranged in a grid with two rows and three columns as shown in the following diagram. Aria, Bella, Charlotte, Danna, and Emma are to be stationed inside the cubicles to work on various company projects.

Find the number of ways of placing the team members in the cubicles in each of the following cases.

1. Each cubicle is large enough to contain the five team members, but Danna and Emma must not be placed in the same cubicle.

2. Each cubicle may only contain one team member. But Aria and Bella must not be placed in cubicles which share a boundary, as they tend to get distracted by each other.

hard

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##### Question 14

calculator

hard

[Maximum mark: 6]

The barcode strings of a new product are created from four letters A, B, C, D and ten digits $0,1,2,\dots,9$. No three of the letters may be written consecutively in a barcode string. There is no restriction on the order in which the numbers can be written.

Find the number of different barcode strings that can be created.

hard

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##### Question 15

calculator

hard

[Maximum mark: 11]

Sophie and Ella play a game. They each have five cards showing roman numerals I, V, X, L, C. Sophie lays her cards face up on the table in order I, V, X, L, C as shown in the following diagram. Ella shuffles her cards and lays them face down on the table. She then turns them over one by one to see if her card matches with Sophie's 4 card directly above. Sophie wins if no matches occur; otherwise Ella wins.

1. Show that the probability that Sophie wins the game is $\dfrac{11}{30}$.

Sophie and Ella repeat their game so that they play a total of $90$ times. Let the discrete random variable $X$ represent the number of times Sophie wins.

1. Determine:
1. the mean of $X$;

2. the variance of $X$. 

hard

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##### Question 16

calculator

hard

[Maximum mark: 25]

Jack and John have decided to play a game. They will be rolling a die seven times. One roll of a die is considered as one round of the game. On each round, John agrees to pay Jack $4 if $1$ or $2$ is rolled, Jack agrees to pay John$2 if $3,4,5$ or $6$ is rolled, and who is paid wins the round. In the end, who earns money wins the game.

1. Show that the probability that Jack wins exactly two rounds is $\dfrac{224}{729}$. 

1. Explain why the total number of outcomes for the results of the seven rounds is $128$.

2. Expand $(1 + y)^7$ and choose a suitable value of $y$ to prove that

$128 = \binom{7}{0} + \binom{7}{1} + \binom{7}{2} + \binom{7}{3} + \binom{7}{4} + \binom{7}{5} + \binom{7}{6} + \binom{7}{7}. \vspace{-0.5em}$
3. Give a meaning of the equality above in the context of the seven
rounds.

1. Find the expected amount of money earned by each player in the game.

2. Who is expected to win the game?

2. Jack and John have decided to play the game again.

1. Find an expression for the probability that John wins five rounds on the first game and two rounds on the second game. Give your answer in the form

$\binom{7}{r}^2\bigg[\frac{1}{3}\bigg]^s\bigg[\frac{2}{3}\bigg]^t \vspace{0.25em}$

where the values of $r,s$ and $t$ are to be found.

2. Use your answer to (d) (i) and seven similar expressions to write down the probability that John wins a total of seven rounds over two games as the sum of eight probabilities.

3. Hence prove that

$\binom{14}{7} = \sum_{k = 0}^7 \binom{7}{k}^2. \vspace{-0.5em}$



3. Now Jack and John roll a die $12$ times. Let $A$ denote the number of rounds Jack wins. The expected value of $A$ can be written as

$\mathrm{E}[A] = \sum_{r=0}^{12} r\binom{12}{r} \left[\dfrac{a^{12-r}}{b^{12}}\right] \vspace{-0.25em}$
1. Find the value of $a$ and $b$.

2. Differentiate the expansion of $(1 + y)^{12}$ to prove that the expected
number of rolls Jack wins is $4$. 

hard

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