1. Time – Time Calculations
Mathematics
Time – Time Calculations

2. Lesson Objectives The aim of this powerpoint is to help you…
Learn the different units for time
Convert between the different units
Undertake calculations involving time

3. Time and Timetables
Of all the measures used in everyday life, time is probably used the most often.
Afterall, you will always want to know when your maths lesson will finish!
Whilst we have generally moved away from imperial units to metric units, it must be remembered that time is NOT metric!

4. Units of Time Here are units of time and their conversion rates:
Second (s)
Minute (m or min) 1 min = 60s
Hour (h) 1 h = 60 min
Day 1 day = 24 h
Week 1 week = 7 days
Fortnight 1 fortnight = 2 weeks or 14 days
Month
Year 1 year = 365 days or 52 weeks
(NB. 1 leap year = 366 days)
Decade 1 decade = 10 years
Century 1 century = 100 years
Millennium 1 millennium = 1000 years

5. Time Conversions
You will either multiply or divide by the conversion rate – use your common sense!
Divide if the units you have and the conversion rate units are the same otherwise multiply.
Changing hours to seconds you will get LOTS of seconds so you will have multiplied by the exchange rate (in this case twice!!!)
5 h  × 60 = 300 mins  × 60 = s
(1 hr = 60 mins) (1 min = 60 secs)

6. Arithmetic and Time
It is much better to use timelines when undertaking calculations involving time but we will look at both methods – traditional arithmetic methods and timeline jumps.
Time is NOT metric! So when moving between the minute and hour columns you can not simply carry 1 or borrow 1!
Remember: 60 mins = 1 h so every 6 in the tens column of the minutes is worth 1 in the units column of the hours and vice versa!

7. Example 1 – Time Line Method
E.g. Add 3 ¼ hours to the time 15:49
First remember that a ¼ hour is the same as ¼ of 60 mins = 15 minutes
15:49
Start at 15:49 and add 3 hours on to 15…
Now add 15 minutes on to 49… remember when it goes past 60 you move into the next hour!
+ 3 h
+ 11 mins then + 4 mins
18:49
19:00
19:04

8. Example 1 – Column Arithmetic
Remember: 60 mins = 1 hr so every 6 in the tens column of the minutes is worth 1 in the units column of the hours!
E.g. Add 3 ¼ hours (i.e. 3h 15m) to the time 15:49
h min
15 : 49
+ 3 : 15
Start on the right = 14 minutes, ie. 4 carry 1
Now we have carry 1 = 6 lots of 10 minutes, ie. carry 1 hour leaving 0 minutes!
Now add the hours together (remembering the carried 1)… 1 9 : 4

9. Example 2 – Time Line Method
E.g. What time is 4½ hours earlier than 11:10?
First remember that a ½ hour is the same as ½ of 60 mins = 30 minutes
6:40
Start at 11:10 and take 4 hours off the 11…
Now take 30 mins off the 10… in 2 jumps is easier!
– 3 h
– 20 mins and – 10 mins
7:00
7:10
11:10

10. Example 2 – Column Arithmetic
Take 4½ hours away from 11:10
h mins
1 1 : 10
– 4 : 30
Starting on the right, 0 – 0 = 0
1 – 3 can’t be done so borrow 1 from the hours column, which is worth 6 (lots of 10 minutes).
6 added on to 1 now gives 7 – 3 = 4
Now in the hour columns we have: 10 – 4 = 6
Remember:
4 ½ hours = 4.5 hours
4.5 hours is NOT 4h 50m!
½ h = ½ of 60 m = 30 mins
4 ½ hours = 4h 30m 6 : 4

11. What next?
Print out the notes called Time3. Read through them and make sure you answer any questions.
Work through the MyMaths lesson and its online homework called Time Calculations found at:
Save and complete the worksheet: TimeCalc-S1.xlsx
Now move on to the SDT powerpoint