S5 ICT | Data Representation and File Size: Tutor Prep Guide (45-minute flow, script, common errors)

2026-11-15

Tutor lesson-prep guides · Secondary ICT

Students read 2 MB as a name, not as a number built from pixels and bits. This lesson turns the unit into a multiplication chain they can run.

Objectives — By the end of the lesson students can: (1) state that one byte is eight bits and that KB, MB and GB step by 1024 rather than 1000; (2) compute 1024 by 768 by 3 bytes as 2.25 MB; (3) compute a three-minute stereo clip as about 30.28 MB from sampling rate, bit depth, channels and seconds; (4) explain that ASCII uses seven bits for 128 codes and why Chinese needs multi-byte encoding.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Multiplication	What is 1024 times 768	786432
The 1024 step	How many KB in 1 MB	1024 KB
Powers of two	What are two to the seven and two to the eight	128 and 256
Division for units	What is 2359296 divided by 1048576	2.25

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Entry check	Ask 1024 times 768, two to the seven, and KB in 1 MB; keep the working on the right of the board	Spoken answers, working on paper
6-16 min	Act one: start from the bit	One bit holds 0 or 1, eight bits make a byte, then B, KB, MB and GB step by 1024	Students state 1 KB is 1024 bytes
16-28 min	Act two: image equals pixels times colour depth	Write 1024 times 768 times 3, multiply pixels then bytes per pixel, divide by 1024 twice	Students reach 2.25 MB
28-38 min	Act three: audio equals four numbers multiplied	Lay 44100, 16, 2 and 180 in four boxes; bits first, then divide by eight	Students reach 30.28 MB
38-43 min	Question types: text file, unit conversion, compression ratio	Same chain with different numbers, three phrasings	Students do one and read out the unit
43-45 min	Exit check	Three spoken questions; note which act each error comes from	All three correct

03 Teaching script (read it out)

Scene 1: Act one: begin with the bit, not with the megabyte

A single bit has two states, 0 or 1, and on its own it cannot hold a letter. So the computer binds eight bits into a byte, which is exactly enough for one English character. Above that the steps are 1024: 1024 bytes make a kilobyte, 1024 kilobytes make a megabyte, 1024 megabytes make a gigabyte. Why 1024 and not 1000? Because the machine counts in binary and 1024 is two to the ten, a clean power. When you see 1 MB, read it as 1048576 bytes, never as one million.

How many bytes are in 1 MB, and why is it not 1000000?

Scene 2: Act two: image size equals width times height times bytes per pixel

A digital photo is a grid of pixels. A width of 1024 and a height of 768 gives 786432 pixels. Each pixel must store a colour, and if each colour channel takes one byte with red, green and blue, that is three bytes per pixel. Multiply the three numbers: 1024 by 768 by 3 gives 2359296 bytes. Divide by 1024 once for 2304 KB, again for 2.25 MB. The chain only ever runs one way: pixels, times colour depth, divided by 1024 twice. Reversed questions work the same way, so given 2 MB and a resolution you divide back up the chain.

If the same 1024 by 768 frame used four bytes per pixel, how large would the file be?

Scene 3: Act three: audio size equals sampling rate times bit depth times channels times seconds

Sound is sampled thousands of times a second. A rate of 44100 means 44100 snapshots each second; a bit depth of 16 means each snapshot records a 16-bit number; stereo means two channels, so multiply by two. Three minutes is 180 seconds. Multiplying the four gives 254016000 bits. Bits are not bytes, so divide by eight for 31752000 bytes, then by 1048576 for 30.28 MB. Divide by eight first because the question normally asks for megabytes, and megabytes are built from bytes, not from bits.

If the same clip were mono, how much smaller would the file be, and why divide by two rather than four?

04 Question types and worked steps

Type	Example	Steps	Answer
Image file size	An 800 by 600 image at three bytes per pixel	Pixels times depth for bytes, then divide by 1024 twice	1440000 bytes, that is 1406.25 KB (about 1.37 MB)
Unit conversion	How many bytes are in 30 MB	Multiply by 1024 twice, never by 1000	31457280 bytes
Audio file size	44.1 kHz, 16-bit, stereo, 60 seconds	44100 times 16 times 2 times 60 for bits, then divide by eight	10584000 bytes, about 10.09 MB
Text file estimate	A plain text file of 2000 English characters	One byte per character in ASCII	2000 bytes, about 1.95 KB

Sanity check (teach this): Three checks: (1) bits become bytes only after dividing by eight, so an answer in bits when megabytes were asked is unfinished; (2) KB and MB step by 1024, and using 1000 understates by about five per cent per step; (3) read the answer back for sense, because 30 MB for three minutes of stereo is plausible while 30 GB is an order of magnitude out.

05 Independent practice (3 questions)

Question	Answer	What to watch
A 640 by 480 image at three bytes per pixel: how large in KB	921600 bytes, that is 900 KB	Check that 1024 was divided once, not twice
A 30-second mono recording at 22050 Hz and eight bits: how many bytes	661500 bytes, about 646 KB	22050 times 8 times 1 times 30 for bits, then divide by eight
How many 2.25 MB photos fit on a 700 MB disc in theory	311 photos	700 divided by 2.25 is 311.1, so take the whole number

06 Common mistakes and what to say

Mistake	Student writes	Say this
Mixing bits and bytes	Answering 254016000 bytes when the chain produced bits	The chain gave you bits; bytes need a division by eight. Circle the unit before writing the answer.
Using 1000 as the step	Saying 1 MB is 1000000 bytes	The machine is binary, so 1024 is the step. With 1000, every later step drifts about 2.4 per cent.
Reading a 24-bit colour as 24 bytes	Multiplying by 24 bytes per pixel	Twenty-four bits is three bytes, so divide by eight first. Bits in the question means bits in the chain.
Forgetting the channel multiplier	A stereo clip computed without the times two	Stereo is two channels, so multiply by two. An answer exactly half the expected size usually means this step is missing.

07 Homework (paper, section and question numbers)

S5 ICT Practice Paper 1, data and systems section — Section A, questions 1 to 4

08 Exit check (last 5 minutes)

Question	Answer	Decision
How many bytes are in 1 KB	1024	Wrong: reteach act one, the 1024 step
How large is a 1024 by 768 image at three bytes per pixel	2.25 MB	Wrong: reteach act two, pixels times depth
How large is three minutes of 44100 Hz, 16-bit stereo	About 30.28 MB	Wrong: reteach act three, the division by eight

09 Tutor FAQ

Which year level is this for?

Hong Kong S5 ICT assesses file size calculation in the data representation unit, and public exam questions often wrap it in images and audio. The S4 work on binary and memory units is the prerequisite, so revise powers of two and the byte steps first if it is missing.

Is 45 minutes enough?

Yes, provided act three gets ten minutes. The commonest slip is skipping the division by eight, so work 254016000 divided by eight on the board and let students press it themselves.

Are the classroom figures official specifications?

No. 44100 Hz, 16-bit and the 1024 step are teaching standards; real devices may use 48000 Hz or compressed formats. What matters is the chain and the direction of the units, not memorising one device.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.