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Maths I

Time calculations

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Calculating time intervals

In the General exam you might be asked to work out how long someone spends at work during the day or how long a train journey lasts. This is calculating a time interval.

For most things we use the decimal system of counting, i.e. hundreds, tens and so on.

Time isn't counted like this because, instead of going up in tens:

  • 60 seconds = 1 minute

  • 60 minutes = 1 hour

Be careful! Lots of people make mistakes when doing time calculations because of this.

There are lots of different methods for calculating time intervals. It doesn't matter which one you choose, as long as you feel confident about using it. However, whichever way you decide to do these questions, show all your working, it helps to stop you getting confused between time and decimal systems of counting.

Here is an example of a typical time question -

Daniel starts school at 8.25am and finishes at 3.15pm. How long does his school day last?

We will go over two ways of doing this, using Time Lines and Counting On.

Time Lines

This is where the time interval is shown on a time line.

First draw a line with 8.25am (when Daniel started school) at one end and 3.15pm (when Daniel finished school) at the other.

A horizontal line showing one time at either end. At the left end it says 8.25am, and at the right side it says 3.15pm.

Next mark 9am and 3pm on the line.

Horizontal line with times of the day written at various points along it. The first time on the left side is 8.25am, followed further by 9am, a longer gap, then 3pm, then 3.15pm.

If you want to, mark in 12 noon.

Horizontal line with 8.25am at the left end, 3.15pm at the right end, and the times 9am, 12 noon and 3pm at the appropriate places along the line.

Now calculate the time interval for each section.

Timeline. Starts at 8.25am, says 35 min, has another mark at 9am, says 3 hours, has another mark at 12 noon, says 3 hours, has another mark at 3pm, says 15 mins, has another mark at 3.15pm and there it ends.

Add them up.

3hrs + 3hrs + 15mins + 35mins = 6hrs50 mins

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