Page 145 - Mathematics Coursebook

15.1 The 12-hour and 24-hour clock

15.1 The 12-hour and 24-hour clock

12
11 1
10 2

9 3

8 4
7 6 5 24-hour clock times are sometimes

written with a colon (:). There may

!ese are di"erent ways to describe the time shown on the clock. be a gap between hours and minutes

of they may be closed up. 03:45,
0345 and 03 45 are all correct ways
It is a quarter to four. It is 45 minutes past 3. of writing the same time.

3 45 am 3 45 pm 03 45 15 35

In the 12-hour clock system, 3 45 am is in the morning and 3 45 pm is in the a$ ernoon.
In the 24-hour clock system, 03 45 is in the morning and 15 45 is in the a$ ernoon.

!e 24-hour system is usually used for timetables, because it is

clear whether the time is in the morning or a$ernoon. 24-hour times always have
four numerals.
You need to understand both the 12-hour and the 24-hour systems.

Worked example 15.1

An aeroplane takes off at 14 40 and lands at 17 25. How long does the journey take?

Method 1
14 40 ¤ 15 40 ¤ 16 40 is 2 hours Count on in hours. Stop at 16 40 because 17 40 is after 17 25.
16 40 ¤ 17 25 is 45 minutes 16 40 to 17 00 is 20 minutes; 17 00 to 17 25 is another 25 minutes.

Time taken is 2 hours 45 minutes

Method 2
14 40 ¤ 15 00 is 20 minutes Count on to the next whole hour. Be careful. You can’t use a
15 00 ¤ 17 00 is 2 hours Count on to the hours fi gure (17). calculator for subtractions like
17 00 ¤ 17 25 is 25 minutes the one in the example.
Time taken is 2 hours 45 minutes 20 minutes + 2 hours + 25 minutes.

144 15 Time