Calculating time intervals - counting on
Counting on
Remember the original question:
Daniel starts school at \(8.25am\) and finishes at \(3.15pm\). How long does his school day last?
This is how to work it out using the 'counting on' method.
Step One
Start at 8.25am and work out the number of minutes until 9am.
\(8.25am - 9am = 35mins\)
Step Two
Next work out how many hours there are between 9am and 3pm.
\(9am - 12noon = 3hrs\)
\(12noon - 3pm = 3hrs\)
Step Three
Next calculate the number of minutes between 3pm and 3.15pm.
\(3pm - 3.15pm = 15mins\)
Step Four
Finally add up the separate time intervals.
\(35mins + 3hrs + 3hrs + 15mins = 6hrs + 50mins = 6hrs 50mins\)
Examples
Question
Amy used the internet from \(10.08\) until \(14.57\). How long was she using the internet?
Add up the time intervals:
\(52mins + 1hr + 2hrs + 57mins = 3hrs + 109mins\)
However \(109mins\) is more than \(60mins\) (one full hour)
\(109mins = 1hr 49mins\)
So we now have \(3hrs + 1hr 49 mins\)
The total time Amy spent on the internet was \(4hrs 49mins\).
Sometimes we have to calculate a time interval across midnight.
Question
Karen travelled on a train overnight. Her journey started at \(21.10\) and ended at \(07.30\). How long did the journey take?
Add up the time intervals:
\(50 mins + 2 hrs + 7 hrs + 30 mins = 9 hrs + 80 mins\)
\(80 mins = 1 hr 20 mins\)
Journey time \(= 9hrs + 1hr 20 mins = 10hrs 20mins\)
