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Class 9 Computer Science — The trail you leave online

Work out how few harmless-looking facts it takes to narrow a crowd down to one person, and tell what a website is handed by you from what it is handed anyway.

Name: ________________________

  1. 1.In a group of 60 people, each year group is shared by 30 of them, each birth month by 5, each admission number by 1, each favourite sport by 12, each pairing of shoe size with birth month by 2, and each email address by 1. Sort each fact by how many people are left once you know it about somebody.

    Groups: Leaves exactly one person · Leaves more than one person

    • Their admission number
    • Their shoe size and birth month together
    • Their birth month
    • Their year group
    • Their favourite sport
    • Their email address

  2. 2.A club has 40 members. Knowing somebody's year group leaves 8 of them. Knowing their birth month as well leaves a quarter of those 8. Mark how many members are left.

    Mark the line with an X.

    012345678910

  3. 3.You visit a website and type nothing into it at all. Which of these does the website learn anyway?

    • a) The name of your school
    • b) The password to your email
    • c) Your date of birth
    • d) The time you arrived

  4. 4.A club has 720 members. Half of them are in the same year group as you. Of those, one in six has a birthday in the same month as you. Of those, one in twelve plays the flute. How many members share all three of those facts with you?

  5. 5.You post a photograph, and three friends save a copy of it before you delete yours. Each of those three then sends it on to four other people, and every one of those people saves a copy too. Once you have deleted your own, how many copies of the photograph are there?

  6. 6.Three facts about you are handed to three different websites, one fact each, and not one of the three is ever told your name. Each fact on its own would fit thousands of people. What has to happen before those three facts can pick out one single person?

    • a) Nothing at all: three facts can never name one person
    • b) Somebody has to put the three facts side by side
    • c) One of the three websites has to be told your name
    • d) All three facts have to be about the same day

  7. 7.In a group of 200 people, each year group is shared by 50 people; inside any one year group each birth month is shared by 5; and inside any one birth month each first initial is shared by 1. Match each set of facts to how many people could still be the person described.

    • Nothing at all is known
    • The year group
    • The birth month, without the year group
    • The year group and the birth month
    • The year group, the birth month and the first initial
    • 20
    • 1
    • 5
    • 50
    • 200

  8. 8.A swimming pool has 1800 members. One in five of them swims before school, and one in three of those is in the same year group as you. How many members fit both of those facts?

Answer key — Class 9 Computer Science — The trail you leave online

  1. 1. Their year group → Leaves more than one person; Their birth month → Leaves more than one person; Their admission number → Leaves exactly one person; Their favourite sport → Leaves more than one person; Their shoe size and birth month together → Leaves more than one person; Their email address → Leaves exactly one person
  2. 2. 2
  3. 3. d) The time you arrived
  4. 4. 5
  5. 5. 15
  6. 6. b) Somebody has to put the three facts side by side
  7. 7. Nothing at all is known → 200; The year group → 50; The birth month, without the year group → 20; The year group and the birth month → 5; The year group, the birth month and the first initial → 1
  8. 8. 120