Topic outline
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Outcomes
In this course you will learn about:
- Rounding off numbers.
- Estimating answers to calculations.
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Numbers can be rounded off, or approximated, to help make calculations simpler. If we need to check calculator workings, or if we do not have a calculator at hand, we can make the calculations simpler by rounding off. We can round up or down to the closest numbers that make mental calculations easier.
Rounding off to the nearest thousand
If the last three digits of a number are less than 499 round down to the nearest whole number thousand. For example, 6345 rounded to the nearest thousand is 6000.
Numbers that have the last three digits of 500 or more should be rounded up to the next thousand. For example, 8629 rounded to the nearest thousand is 9000.
Rounding off using significant figures
We generally round off to a certain number of significant figures.
There are a few rules to be aware of when rounding off using significant figures, these are discussed in the following video.
After watching the video try ALL THREE of the activities below to test your understanding of rounding off. -
To check your calculations and to see how reasonable your answer is you can estimate the answer before doing the calculation. If your calculated answer is wrong, you will see straight away that it is not close to your estimate. We use the symbol \(\approx\) to show that the answer is an approximate value.
Example
Sindy and TK both enter \(49\times 105\) into the calculator and get different answers. Sindy gets an answer of \( 5 040\) while TK gets an answer of \( 7 350\). Using estimation, who is closer to the actual answer?
Round off \(49\) to \(50\) and \(105\) to \(100\).
\(50\times 100=5 000\). So, \( 49\times 105 \approx 5 000\).
Therefore, Sindy has the closest answer.
For more examples on approximating answers watch the next video.