Estimate answers to calculations
Estimating is actually very easy, so long as you don't over complicate it. Its really just a two step process:

ROUND EVERYTHING OFF to nice easy CONVENIENT NUMBERS

Then WORK OUT THE ANSWER using these nice easy numbers....that's it!
Let's go through a practice question.
'Estimate the answer to the following equation: 647 / (4.8×9.7)'
In this case, when they ask you to estimate, they actually want you to round the equation off to each number's nearest significant figure. 647 can be rounded up to 650, a much easier number for calculations, and 4.8 and 9.7 can be rounded up to 5 and 10 respectively, making the equation...
650 / (5×10)
Which should now be much easier to do without a calculator!
Note: Remember when they ask you to estimate an answer, they are simply asking you to make the question and answer simpler!
Follow the links below to see how this topic has appeared in past exam papers
AQA Unit 2 November 2012 (H)  Page 2, Question 1
AQA Unit 2 March 2011 (H)  Page 6, Question 7
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