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Grade 9 Computer Science — Working out a file size

Work out how many bytes a picture, a piece of text or a recording takes up, from how many things are in it and how many bytes one of those things needs.

Name: ________________________

  1. 1.A recording stores 16000 samples every second, each sample taking 2 bytes, and it lasts 3 seconds. How many bytes does the recording take?

  2. 2.One picture is 100 pixels by 80 pixels with 1 byte per pixel. Another is 50 pixels by 40 pixels with 3 bytes per pixel. Which is larger, and by how much?

    • a) The first, by 4000 bytes
    • b) The first, by 2000 bytes
    • c) The two are exactly the same size
    • d) The second, by 2000 bytes

  3. 3.A picture is 40 pixels wide and 30 pixels tall, and 1 byte is stored for every pixel. How many bytes does the picture take?

  4. 4.Match each file to the number of bytes it takes.

    • 24 pixels by 25 pixels, 1 byte per pixel
    • 1500 characters, 1 byte each
    • 30 pixels by 30 pixels, 2 bytes per pixel
    • 400 samples a second for 6 seconds, 1 byte each
    • 20 pixels by 15 pixels, 3 bytes per pixel
    • 1500
    • 900
    • 600
    • 1800
    • 2400

  5. 5.A picture is 64 pixels wide and 32 pixels tall, with 1 byte per pixel, and one kilobyte is 1024 bytes. How many kilobytes is the picture?

  6. 6.Put these files in order of size, smallest first.

    • 500 samples a second for 5 seconds, 1 byte each
    • 20 pixels by 20 pixels, 3 bytes per pixel
    • 45 pixels by 40 pixels, 1 byte per pixel
    • 800 characters, 1 byte each

  7. 7.A color picture is 200 pixels wide and 150 pixels tall, and 3 bytes are stored for every pixel. How many bytes does it take?

  8. 8.A picture takes 7200 bytes altogether and stores 2 bytes for every pixel. How many pixels does the picture have?

Answer key — Grade 9 Computer Science — Working out a file size

  1. 1. 96000
  2. 2. b) The first, by 2000 bytes
  3. 3. 1200
  4. 4. 24 pixels by 25 pixels, 1 byte per pixel → 600; 1500 characters, 1 byte each → 1500; 30 pixels by 30 pixels, 2 bytes per pixel → 1800; 400 samples a second for 6 seconds, 1 byte each → 2400; 20 pixels by 15 pixels, 3 bytes per pixel → 900
  5. 5. 2
  6. 6. 1. 800 characters, 1 byte each 2. 20 pixels by 20 pixels, 3 bytes per pixel 3. 45 pixels by 40 pixels, 1 byte per pixel 4. 500 samples a second for 5 seconds, 1 byte each
  7. 7. 90000
  8. 8. 3600