9.2 Mass, Time & Elapsed Time
Key Takeaways
- Mass converts in thousands: 1 t = 1000 kg and 1 kg = 1000 g — always match units before adding or comparing masses
- On an analogue clock the short hand shows the hour and the long hand shows minutes in groups of five around the dial
- 24-hour time runs from 00:00 to 23:59; to convert a PM time, add 12 to the hour — 3:45 pm is 15:45
- 1 h = 60 min and 1 min = 60 s, so time conversions use 60, not 100 — 2.5 h is 150 min, not 250 min
- Elapsed time is safest computed by bridging to the next whole hour: from 9:50 to 11:15 is 10 min + 1 h + 15 min = 1 h 25 min
Mass and time questions look simple but punish students who treat all measurement the same way. Mass behaves like every other metric unit — thousands all the way — while time refuses to play by tens: it uses 60 seconds, 60 minutes and 24 hours. ICAS writers know exactly where students slip, and this section covers both.
Mass Units
| Unit | Symbol | Relationship | Typical object |
|---|---|---|---|
| gram | g | base unit | a paperclip ≈ 1 g |
| kilogram | kg | 1 kg = 1000 g | a bag of flour = 1 kg |
| tonne | t | 1 t = 1000 kg | a small car ≈ 1 t |
Worked example: A grocer packs 3.5 kg of potatoes into 500 g bags. How many full bags? Convert first: 3.5 kg = 3500 g. Then 3500 ÷ 500 = 7 bags.
Reading scales is a partner skill. On a kitchen scale with marks every 50 g between 200 g and 300 g, a needle three marks past 200 reads 350 g. Always work out what one small division is worth before reading: (300 − 200) ÷ number of gaps.
Telling Time: Analogue and Digital
On an analogue clock the short hand is the hour and the long hand counts minutes in fives: pointing at the 3 means 15 minutes past, at the 7 means 35 minutes past. 'Quarter past' is :15, 'half past' is :30, 'quarter to' is :45 of the next hour.
12-Hour vs 24-Hour Time
24-hour time removes the am/pm ambiguity and is the language of timetables.
| 12-hour | 24-hour |
|---|---|
| 12:00 am (midnight) | 00:00 |
| 8:30 am | 08:30 |
| 12:00 pm (noon) | 12:00 |
| 1:15 pm | 13:15 |
| 3:45 pm | 15:45 |
| 11:59 pm | 23:59 |
Rule: for PM times after noon, add 12 to the hour. To go backwards, subtract 12 and label it pm. Midnight is the special case — 00:00.
Converting Time Units
- 1 minute = 60 seconds
- 1 hour = 60 minutes
- 1 day = 24 hours
- 1 week = 7 days
The classic trap: because the metric system trains you to use 100, students write 2.5 h = 250 min. Wrong. 2.5 h = 2 × 60 + 0.5 × 60 = 120 + 30 = 150 min. Similarly, 100 minutes is 1 h 40 min, not 1 hour exactly.
Elapsed Time: The Bridge Method
The safest no-calculator technique is to bridge to the next whole hour, then add the pieces.
Worked example: A film starts at 9:50 am and ends at 11:15 am. How long is it?
- 9:50 → 10:00 is 10 minutes
- 10:00 → 11:00 is 1 hour
- 11:00 → 11:15 is 15 minutes
- Total: 10 + 60 + 15 = 85 minutes = 1 h 25 min
Across days: A bus departs at 21:40 and arrives at 06:25 the next morning. From 21:40 to midnight is 2 h 20 min; from midnight to 06:25 is 6 h 25 min. Total = 8 h 45 min.
Timetables and Calendars
Timetable questions give a table of departure times and ask for waiting time or journey length — pure elapsed-time work once you find the right row. Calendar questions need the month lengths: 30 days hath September, April, June and November; all the rest have 31, except February with 28 (29 in a leap year — divisible by 4).
Worked example: How many days from 15 March to 10 April? March has 31 days, so 31 − 15 = 16 days left in March, plus 10 days of April = 26 days.
Extension for older papers: time zones. If Sydney is 2 hours ahead of Perth, a 3:00 pm call in Sydney happens at 1:00 pm in Perth — add going east, subtract going west.
More Worked Examples in the ICAS Style
Reading a scale precisely
A bathroom scale shows kilograms with four small marks between 60 kg and 70 kg. Four marks create five spaces, so each space is worth (70 − 60) ÷ 5 = 2 kg. If the pointer sits on the third mark past 60, it reads 60 + 3 × 2 = 66 kg. The single most common scale error is dividing by the number of marks instead of the number of spaces.
A timetable problem
A bus leaves the school at 8:12 am, 8:42 am and 9:12 am. The ride takes 25 minutes. You arrive at the stop at 8:30 am. When do you reach the library? You just miss the 8:12 bus, so you catch the 8:42 and arrive at 8:42 + 25 min = 9:07 am. Note the total elapsed time from arriving at the stop (37 minutes) differs from the journey time (25 minutes) — ICAS asks for both, so read the question twice.
Working backwards
A cake must come out of the oven at 4:10 pm and bakes for 1 h 45 min. When should it go in? Bridge backwards: 4:10 minus 10 min is 4:00, minus 1 h is 3:00, minus the remaining 35 min is 2:25 pm. Reversing direction is where many students add instead of subtract.
am and pm
The abbreviations come from Latin: ante meridiem (before midday) and post meridiem (after midday). Noon is 12:00 pm and midnight is 12:00 am — a pair worth memorising because the 24-hour equivalents 12:00 and 00:00 confuse almost everyone at first.
Leap years
A year is a leap year if it divides by 4 (2024, 2028), except century years, which need to divide by 400 (2000 was a leap year; 1900 was not). For ICAS purposes, knowing that February has 29 days in a leap year is usually enough.
A train leaves at 10:48 am and arrives at 12:23 pm. How long is the journey?
A recipe book says a roast needs 2.4 hours in the oven. How many minutes is that?