Time
Measurement · Time
CBC Grade 5 Mathematics — Time
Lesson Objectives
By the end of this topic, a learner should be able to:
- Identify the second as a unit for measuring time
- Explain the relationship between minutes and seconds
- Convert between minutes and seconds
- Add, subtract, multiply, and divide time given in minutes and seconds
- Appreciate the use of minutes and seconds in everyday situations
Time
Time is measured in hours, minutes and seconds. In Grade 5 we work with the second.
a) The second
A second is a very short time. Say "one thousand" out loud and that takes about one second.
1 minute = 60 seconds.
Sixty, not a hundred. This catches many people out. Most of our other units go in tens and hundreds.
b) Where seconds are used
A race is timed in seconds. So is boiling an egg, or holding your breath.
| Activity | About |
|---|---|
| One breath | 4 seconds |
| Washing hands | 20 seconds |
| A 100 m race | 15 seconds |
| Boiling an egg | 300 seconds |
c) Converting
To change minutes to seconds, multiply by 60.
To change seconds to minutes, divide by 60.
A check: seconds are smaller, so there are more of them.
Take care with the leftover. 150 seconds is 2 minutes and 30 seconds. Written that way it is not 2.5 minutes.
d) Adding
Add the seconds first, then the minutes.
The seconds may come to 60 or more. Change each 60 into a minute and carry it over.
Remember it is 60, not 100. Carrying at 100 is the commonest mistake here.
e) Subtracting
If there are not enough seconds, borrow one minute and turn it into 60 seconds.
Take 1 min 45 s from 4 min 20 s. Borrow a minute so the 20 becomes 80. Then 80 − 45 = 35 seconds, and 3 − 1 = 2 minutes. The answer is 2 min 35 s.
f) Multiplying and dividing
Multiply or divide each part, then tidy up.
Three laps of 1 min 30 s take 3 min 90 s. That is 4 min 30 s.
A 6-minute job split between 4 people takes 1 min 30 s each.
g) Where this is used
Timing a race. Cooking. Catching a bus. Timing how long a task takes at school.
:::checkpoint Check yourself
- How many seconds make one minute?
- Change 5 minutes into seconds.
- Change 200 seconds into minutes and seconds.
- Add 2 min 50 s and 1 min 30 s. :::
Definitions
- Minute — a unit of time equal to 60 seconds.
- Second — the smaller base unit of time in this topic.
Must Remember
- 1 minute = 60 seconds, not 1000.
- Convert mixed minute/second amounts to seconds before calculating.
- Convert the final answer back if needed.
- Multiply to convert minutes to seconds; divide to convert seconds to minutes.
Conclusion
Same method as Length, Capacity, Mass, but the conversion factor is 60, not 1,000.