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Grade 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.A club has 600 members. Put these facts in order of how many members each one leaves, fewest first.

    • Plays the cello, which one member in every twenty-five does
    • Born in the same week as you, which one member in every hundred is
    • Travels by bus, which one member in every five does
    • Is in the same year group as you, which one member in every two is

  2. 2.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?

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

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

  4. 4.Sort each of these by whether a website is told it even when you type nothing into the page.

    Groups: Told even if you type nothing · Told only if you type it in

    • Your date of birth
    • The time you arrived
    • Your full name
    • The address your computer is using
    • The name of your school
    • Which browser you are using

  5. 5.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?

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

  7. 7.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?

  8. 8.One fact leaves 900 people who could be the person meant. A second fact cuts that down by a factor of 30, and a third cuts what is left by a factor of 6. How many people are still left?

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

  1. 1. 1. Born in the same week as you, which one member in every hundred is 2. Plays the cello, which one member in every twenty-five does 3. Travels by bus, which one member in every five does 4. Is in the same year group as you, which one member in every two is
  2. 2. 15
  3. 3. a) The time you arrived
  4. 4. The time you arrived → Told even if you type nothing; Which browser you are using → Told even if you type nothing; The address your computer is using → Told even if you type nothing; Your full name → Told only if you type it in; The name of your school → Told only if you type it in; Your date of birth → Told only if you type it in
  5. 5. 5
  6. 6. d) Somebody has to put the three facts side by side
  7. 7. 120
  8. 8. 5