Page 144 - Mathematics Coursebook

15 Time

Nowadays the units used for measuring are almost always decimal, Key words
Key words
Key words
based on 10, 100 or 1000. For example, there are 100 cents in a
dollar, 10 millimetres in a centimetre and 1000 grams in a kilogram. Make sure you learn and
Using decimal systems makes calculation easy. understand these key words:
!ere is an important exception to this. Time is not measured in time

a decimal system. !e system of measuring time, in hours and 12-hour clock

minutes, is very ancient. !ere are 60 minutes in an hour, 24 hours 24-hour clock
in a day and 7 days in a week. !is can make calculations more

awkward.
Although some countries may still use di"erent units for some measures, the passage of time is one

thing that everyone in the world measures in exactly the same way. It would make life very di#cult if
that were not the case!
Of course the actual time varies in di"erent countries. For example, because of the Earth turning, it is

evening in Australia while it is midday in the UK. !is is something you need to remember if you travel

from one country to another, or you want to talk on a phone or computer to someone in a di"erent country.

!ere are two ways of writing time. !ese are the 12-hour clock system and the 24-hour clock system.
In everyday conversation, people generally use the 12-hour system. Timetables almost always use the
24-hour system to avoid confusion.

Alarm clock (12-hour system). Digital clock (24-hour system). Astronomical clock.

y

2.0
Distance from home (km) 1.5

1.0

0.5

0 x
0 10 20 30 40
Time (minutes)

Graph shows a walk to school (distance travelled against time).
15 Time 143