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?
1 h 25 min
1 h 35 min
2 h 25 min
1 h 45 min
A recipe book says a roast needs 2.4 hours in the oven. How many minutes is that?
124 minutes
204 minutes
224 minutes
144 minutes
Sections you finish are checked off in the contents.