Combinations & Selection

135 questions · 21 question types identified

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Basic committee/group selection

Select a committee or group of a fixed size from a larger population with no restrictions or simple restrictions like 'must include person X' or 'cannot include both X and Y together'.

21 Easy -1.0
15.6% of questions
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Find the value of \(\frac{100!}{98! \times 3!}\) Circle your answer. [1 mark] \(\frac{50}{147}\) \quad \(1650\) \quad \(3300\) \quad \(161700\)
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Easiest question Easy -2.0 »
8
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    \(5 ( \mathrm { v } )\)
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    Hardest question Standard +0.8 »
    7
    1. List the 15 partitions of the set \(\{ \mathrm { A } , \mathrm { B } , \mathrm { C } , \mathrm { D } , \mathrm { E } \}\) in which A and E are in the same subset.
    2. By considering the number of subsets in each of the partitions in part (a), or otherwise, explain why there are 8 partitions of the set \(\{ \mathrm { A } , \mathrm { B } , \mathrm { C } , \mathrm { D } , \mathrm { E } \}\) into two subsets with A and E in different subsets. Ali says that each of the 15 partitions from part (a) can be used to give two partitions in which A and E are in different subsets by moving E into a subset on its own or by moving E into another subset.
      [0pt]
      1. By considering the partition from part (a) into just one subset, show that Ali is wrong. [1]
      2. By considering a partition from part (a) into more than two subsets, show that Ali is wrong.
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    Multi-stage selection problems

    Select groups in multiple stages or with multiple constraints that must all be satisfied simultaneously, often requiring case-by-case analysis of different composition scenarios.

    21 Standard +0.5
    15.6% of questions
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    2 A group of 6 people is to be chosen from 4 men and 11 women.
    1. In how many different ways can a group of 6 be chosen if it must contain exactly 1 man?
      Two of the 11 women are sisters Jane and Kate.
    2. In how many different ways can a group of 6 be chosen if Jane and Kate cannot both be in the group?
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    Easiest question Moderate -0.3 »
    8 Freddie has 6 toy cars and 3 toy buses, all different. He chooses 4 toys to take on holiday with him.
    1. In how many different ways can Freddie choose 4 toys?
    2. How many of these choices will include both his favourite car and his favourite bus?
      Freddie arranges these 9 toys in a line.
    3. Find the number of possible arrangements if the buses are all next to each other.
    4. Find the number of possible arrangements if there is a car at each end of the line and no buses are next to each other.
      If you use the following lined page to complete the answer(s) to any question(s), the question number(s) must be clearly shown.
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    Hardest question Challenging +1.2 »
    4 A cricket team of 11 players is to be chosen from 21 players consisting of 10 batsmen, 9 bowlers and 2 wicketkeepers. The team must include at least 5 batsmen, at least 4 bowlers and at least 1 wicketkeeper.
    1. Find the number of different ways in which the team can be chosen. Each player in the team is given a present. The presents consist of 5 identical pens, 4 identical diaries and 2 identical notebooks.
    2. Find the number of different arrangements of the presents if they are all displayed in a row.
    3. 10 of these 11 presents are chosen and arranged in a row. Find the number of different arrangements that are possible.
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    Committee with gender/category constraints

    Select a committee or group where the composition must satisfy constraints on the number of people from different categories (e.g., men/women, year groups, instrument types) such as 'at least 2 men', 'more women than men', or 'exactly 3 from each category'.

    12 Moderate -0.1
    8.9% of questions
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    1 A bag contains 12 marbles, each of a different size. 8 of the marbles are red and 4 of the marbles are blue. How many different selections of 5 marbles contain at least 4 marbles of the same colour?
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    Easiest question Moderate -0.8 »
    5 A committee of 5 people is to be chosen from 6 men and 4 women. In how many ways can this be done
    1. if there must be 3 men and 2 women on the committee,
    2. if there must be more men than women on the committee,
    3. if there must be 3 men and 2 women, and one particular woman refuses to be on the committee with one particular man?
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    Hardest question Standard +0.8 »
    4 In a music competition, there are 8 pianists, 4 guitarists and 6 violinists. 7 of these musicians will be selected to go through to the final. How many different selections of 7 finalists can be made if there must be at least 2 pianists, at least 1 guitarist and more violinists than guitarists?
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    Probability of specific committee composition

    Calculate the probability that a randomly selected committee or group has a specific composition (e.g., exactly k members from category A) using combinations and probability ratios.

    10 Moderate -0.4
    7.4% of questions
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    2 A committee of 6 people is to be chosen at random from 7 men and 9 women. Find the probability that there are no men on the committee.
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    Easiest question Moderate -0.8 »
    2 A committee of 6 people is to be chosen at random from 7 men and 9 women. Find the probability that there are no men on the committee.
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    Hardest question Standard +0.8 »
    3
    1. Alex places 20 black counters and 8 white counters into a bag. She removes 8 counters at random without replacement. Find the probability that the bag now contains exactly 5 white counters.
    2. Bill arranges 8 blue counters and 4 green counters in a random order in a straight line. Find the probability that exactly three of the green counters are next to one another.
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    Implementing simple random or systematic sampling

    Describe or carry out the practical steps to obtain a simple random sample or systematic sample, including use of random number tables, numbered lists, or dice.

    9 Easy -1.6
    6.7% of questions
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    2 Describe briefly how to use random numbers to choose a sample of 10 students from a year-group of 276 students.
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    Easiest question Easy -1.8 »
    2 Describe briefly how to use random numbers to choose a sample of 10 students from a year-group of 276 students.
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    Hardest question Moderate -0.8 »
    2 A school has 900 pupils. For a survey, Jan obtains a list of all the pupils, numbered 1 to 900 in alphabetical order. She then selects a sample by the following method. Two fair dice, one red and one green, are thrown, and the number in the list of the first pupil in the sample is determined by the following table.
    \cline { 3 - 8 } \multicolumn{2}{c|}{}Score on green dice
    \cline { 3 - 8 } \multicolumn{2}{c|}{}123456
    Score on
    red dice
    1,2 or 3123456
    For example, if the scores on the red and green dice are 5 and 2 respectively, then the first member of the sample is the pupil numbered 8 in the list. Starting with this first number, every 12th number on the list is then used, so that if the first pupil selected is numbered 8 , the others will be numbered \(20,32,44 , \ldots\).
    1. State the size of the sample.
    2. Explain briefly whether the following statements are true.
      1. Each pupil in the school has an equal probability of being in the sample.
      2. The pupils in the sample are selected independently of one another.
      3. Give a reason why the number of the first pupil in the sample should not be obtained simply by adding together the scores on the two dice. Justify your answer.
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    Selection from categorized items

    Select items from multiple categories (e.g., books, songs, exam questions from different sections) where specific numbers must be chosen from each category.

    8 Moderate -0.7
    5.9% of questions
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    2 An examination paper consists of two sections. Section A has 5 questions and Section B has 9 questions. Candidates are required to answer 6 questions.
    1. In how many different ways can a candidate choose 6 questions, if 3 are from Section A and 3 are from Section B?
    2. Another candidate randomly chooses 6 questions to answer. Find the probability that this candidate chooses 3 questions from each section.
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    Critique of sampling methods

    Identify problems with a proposed sampling method, explain why it would not produce a random or representative sample, or suggest improvements to a flawed sampling procedure.

    7 Easy -1.6
    5.2% of questions
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    1 It is desired to obtain a random sample of 15 pupils from a large school. One pupil suggests listing all the pupils in the school in alphabetical order and choosing the first 15 names on the list.
    1. Explain why this method is unsatisfactory.
    2. Suggest a better method.
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    Describing or comparing sampling methods

    Explain, describe, or compare sampling methods (simple random, stratified, systematic, quota, opportunity/convenience) including their advantages, disadvantages, and potential biases.

    6 Easy -2.0
    4.4% of questions
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    11 A survey is undertaken to find out the most popular political party in London.
    The first 1100 available people from London are surveyed.
    Identify the name of this type of sampling.
    Circle your answer.
    simple random
    opportunity
    stratified
    quota
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    Calculating stratified sample sizes

    Calculate the number of individuals to sample from each stratum in a stratified sample, given the stratum sizes and total sample size.

    6 Easy -1.5
    4.4% of questions
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    1. A gym club has 400 members of which 300 are males.
    Explain clearly how a stratified sample of size 60 could be taken.
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    Probability distributions from selection

    Derive the complete probability distribution for a random variable defined by a selection process (e.g., number of vowels in selected letters, sum of numbers on selected cards).

    5 Moderate -0.0
    3.7% of questions
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    3 A team of 4 is to be randomly chosen from 3 boys and 5 girls. The random variable \(X\) is the number of girls in the team.
    1. Draw up a probability distribution table for \(X\).
    2. Given that \(\mathrm { E } ( X ) = \frac { 5 } { 2 }\), calculate \(\operatorname { Var } ( X )\).
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    Repeated trials with selection

    Calculate probabilities for outcomes over multiple independent trials of a selection process (e.g., probability that an event occurs exactly k times in n trials).

    5 Standard +0.4
    3.7% of questions
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    2 Every evening, 5 men and 5 women are chosen to take part in a phone-in competition. Of these 10 people, exactly 3 will win a prize. These 3 prize-winners are chosen at random.
    1. Find the probability that, on a particular evening, 2 of the prize-winners are women and the other is a man. Give your answer as a fraction in its simplest form.
    2. Four evenings are selected at random. Find the probability that, on at least three of the four evenings, 2 of the prize-winners are women and the other is a man.
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    Pigeonhole principle applications

    Use the pigeonhole principle to prove that certain configurations must exist when distributing items into containers or selecting from constrained sets.

    4 Standard +0.5
    3.0% of questions
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    2 Mo eats exactly 6 doughnuts in 4 days.
    1. What does the pigeonhole principle tell you about the number of doughnuts Mo eats in a day? Mo eats exactly 6 doughnuts in 4 days, eating at least 1 doughnut each day.
    2. Show that there must be either two consecutive days or three consecutive days on which Mo eats a total of exactly 4 doughnuts. Mo eats exactly 3 identical jam doughnuts and exactly 3 identical iced doughnuts over the 4 days.
      The number of jam doughnuts eaten on the four days is recorded as a list, for example \(1,0,2,0\). The number of iced doughnuts eaten is not recorded.
    3. Show that 20 different such lists are possible.
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    Selection with family/relationship restrictions

    Select a group where certain individuals have special relationships (siblings, twins, couples) and restrictions apply to whether they can both be included or excluded together.

    3 Moderate -0.1
    2.2% of questions
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    1 A committee of 5 people is to be chosen from 4 men and 6 women. William is one of the 4 men and Mary is one of the 6 women. Find the number of different committees that can be chosen if William and Mary refuse to be on the committee together.
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    Arrangements in multiple rows/groups

    Arrange people into multiple distinct rows, groups, or positions (e.g., front row and back row, different taxis, different bins) where the groups themselves may be distinguishable or indistinguishable.

    3 Standard +0.3
    2.2% of questions
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    7
    1. A group of 6 teenagers go boating. There are three boats available. One boat has room for 3 people, one has room for 2 people and one has room for 1 person. Find the number of different ways the group of 6 teenagers can be divided between the three boats.
    2. Find the number of different 7-digit numbers which can be formed from the seven digits 2, 2, 3, 7, 7, 7, 8 in each of the following cases.
      1. The odd digits are together and the even digits are together.
      2. The 2 s are not together.
        If you use the following lined page to complete the answer(s) to any question(s), the question number(s) must be clearly shown.
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    Probability with replacement/sequential selection

    Calculate probabilities when items are selected sequentially with or without replacement, often involving conditional probabilities or tree diagrams for multiple selections.

    3 Moderate -0.8
    2.2% of questions
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    2 Thomas has six tiles, each with a different letter of his name on it.
    1. Thomas arranges these letters in a random order. Find the probability that he arranges them in the correct order to spell his name.
    2. On another occasion, Thomas picks three of the six letters at random. Find the probability that he picks the letters T, O and M (in any order).
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    Arranging identical items in a line

    Arrange identical items of different types in a line or row, counting distinct arrangements (e.g., arranging identical packets of different products on a shelf, arranging identical colored items in sequence).

    3 Standard +0.3
    2.2% of questions
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    5 A game is played with cards, each of which has a single digit printed on it. Eleanor has 7 cards with the digits \(1,1,2,3,4,5,6\) on them.
    1. How many different 7-digit numbers can be made by arranging Eleanor's cards?
    2. Eleanor is going to select 5 of the 7 cards and use them to form a 5 -digit number. How many different 5-digit numbers are possible?
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    Bin packing problems

    Pack items (often with given weights, sizes, or lengths) into bins or containers with capacity constraints using algorithms like first-fit, first-fit decreasing, or finding optimal packings.

    3 Standard +0.3
    2.2% of questions
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    6 The following masses, in kg, are to be packed into bins. $$\begin{array} { l l l l l l l l l l } 8 & 5 & 9 & 7 & 7 & 9 & 1 & 3 & 3 & 8 \end{array}$$
    1. Chloe says that first-fit decreasing gives a packing that requires 4 bins, but first-fit only requires 3 bins. Find the maximum capacity of the bins. First-fit requires one pass through the list and the time taken may be regarded as being proportional to the length of the list. Suppose that shuttle sort was used to sort the list into decreasing order.
    2. What can be deduced, in this case, about the order of the time complexity, \(\mathrm { T } ( n )\), for first-fit decreasing?
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    Arrangements with adjacency constraints

    Arrange people or objects in a line or row where certain individuals must be next to each other, or must not be next to each other, or specific groups must stand together.

    2 Standard +0.6
    1.5% of questions
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    The members of a team stand in a random order in a straight line for a photograph. There are four men and six women.
    1. Find the probability that all the men are next to each other. [3]
    2. Find the probability that no two men are next to one another. [4]
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    Conditional probability in selection

    Calculate conditional probabilities or determine independence/exclusivity of events defined by selection outcomes (e.g., probability of event A given event B occurred).

    2 Standard +0.3
    1.5% of questions
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    7 Rory has 10 cards. Four of the cards have a 3 printed on them and six of the cards have a 4 printed on them. He takes three cards at random, without replacement, and adds up the numbers on the cards.
    1. Show that P (the sum of the numbers on the three cards is \(11 ) = \frac { 1 } { 2 }\).
    2. Draw up a probability distribution table for the sum of the numbers on the three cards. Event \(R\) is 'the sum of the numbers on the three cards is 11 '. Event \(S\) is 'the number on the first card taken is a \(3 ^ { \prime }\).
    3. Determine whether events \(R\) and \(S\) are independent. Justify your answer.
    4. Determine whether events \(R\) and \(S\) are exclusive. Justify your answer.
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    Arrangements of letters with restrictions

    Arrange letters of a word or set of distinct/identical letters with constraints such as specific letters in specific positions, certain letters together, or letters in alphabetical order.

    1 Moderate -0.3
    0.7% of questions
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    The letters of the word 'SEPARATE' are to be rearranged. Find the probability that, in a randomly chosen rearrangement, the two letters 'A' are not next to each other. [4]
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    Counting with digit/number constraints

    Count how many numbers can be formed from a set of digits subject to constraints like being even, being in a certain range, having specific digits in specific positions, or using distinct digits.

    1 Moderate -0.8
    0.7% of questions
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    8.
    1. A group of four different letters is chosen from the alphabet of 26 letters, regardless of order.
      1. How many different groups can be chosen?
      2. Find the probability that a randomly chosen group includes the letter P .
    2. A three-digit number greater than 100 is formed using three different digits from the ten digits \(0,1,2,3,4,5,6,7,8,9\).
      1. Show that 648 different numbers can be formed. One of these 648 numbers is chosen at random.
      2. Find the probability that all three digits in the number are even. (You are reminded that 0 is an even number.)
      3. Find the probability that the number is even.
        [0pt]
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