Gcse maths: rounding numbers

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Decimal places

The last digit of the number of decimal places you're given, the number next to the last is called the decider if the decider is 5 or above the last digit will move one up if it's 4 or less it will stay the same.

Put into practise: What is 7.45839 to 2 decimal places?
= 7.46 because the decider is 8

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Trickier cases with nines

1) If you have to round up a 9 (to 10), replace the 9 with 0, and carry 1 to the left.
2) Remember to keep enough zeros to fill the right number of decimal places.

Pit into practise 1: Round 45.699 to 2 d.p.
45.699 - 45.70 to 2 d.p. this is because the decider was 9 and the last number was 9 which means that becomes zero and carries 1 to the left however, there are two decimal places included because that is what the question required.

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Significant figures

The 1st 2nd 3rd and 4th significant figure are the first numbers in the set of numbers.
If a question asks you to round up to 2 s.f. Find e first two numbers and after that second number the number after that will be the decider and round up the second number as show in decimals.
1) After the s.f. There can not be any numbers or zeros after significant figures.

Put into practise: what's 54.7651 to 3 s.f. To 2 s.f. To 1 s.f.?

To 3 s.f. = 54.8

To 2 s.f. = 55

To 1 s.f. = 50

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Estimating

1) Round everything off to nice, easy, convenient numbers.
2) Then work out the answer using these nice easy numbers — that's it!

Put into practise: Estimate the value of 127.8 + 41.9 / 56.5 X 3.2, showing all your working out.

1) Round all the numbers to easier ones - 1 or 2 s.f. Usually does the trick.
127.8 + 41.9 / 56.5 X 3.2 — 130 + 40/ 60 X 30

170/180 = 1

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