Time, Currency & Units

Summary — What's in This Chapter

• Time: conversions (seconds → minutes → hours → days → years), reading 12-hour vs 24-hour clocks, calculating durations, timetables, and time zones.
• Money calculations: spotting keywords (total, difference, exchange), and rounding money correctly — usually to 2 decimal places.
• Exchange rates: converting between currencies by multiplying or dividing by the given rate.
• Converting units: length, mass, and volume/capacity conversions using powers of 10.
• Squared & cubic units: why you must square or cube the conversion factor for area and volume — not just multiply once.

1. Time

Why time is "annoying" compared to normal numbers

Every other measurement in maths (length, mass, money) is based on powers of 10 — that's the decimal system. Time is not decimal. It's a leftover from ancient Babylonian counting, which is why we have 60 seconds in a minute (not 100), and 24 hours in a day (not 10). This is exactly why time trips people up on calculators — more on that in the Exam Tips section.

12-hour clock vs 24-hour clock

Think of it like this: a 12-hour clock is like running around a circular track twice a day — once labelled AM (midnight to midday) and once labelled PM (midday to midnight). A 24-hour clock only runs around the track once, so it needs bigger numbers (00:00 to 23:59) instead of relabelling with AM/PM.

• 24-hour time always uses four digits, e.g. 09:05, not 9:5.
• Midnight = 00:00 (24hr) = 12:00am (12hr)
• Midday = 12:00 (24hr) = 12:00pm (12hr)
• Anything from 1pm onwards: add 12 to the hour. E.g. 5:32pm → 17:32.

Reading an analogue clock

An analogue clock always works in 12-hour time. The short hand points to the hour (each number = 1 hour). The long hand points to the minutes (each number = 5 minutes, since 12 × 5 = 60). Some clocks add small tick marks between the numbers for individual minutes.

Calculating durations — the "chunks of time" method

not