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1.1 Number and rounding

Number and rounding

Everything on number and rounding that can come up in the WJEC GCSE Mathematics and Numeracy, in all three units. Each skill below has the method, the mistake that costs the most marks, a worked example, and free practice marked the way WJEC marks it.

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A worked example

Rounding · [1 mark]

Round 51.607 to 2 decimal places.

  1. B151.61

The exam skills (6)

Rounding

  1. Count the decimal places you need and draw a line after that digit.
  2. Look at the next digit: 5 or more rounds up, 4 or less stays.
  3. Write exactly that many decimal places, even if the last one is 0.

Watch out: Dropping a final zero. 3.40 to 2 decimal places must be written 3.40.

Worked example · Practise this skill free

Significant figures

  1. The first significant figure is the first digit that is not zero.
  2. Count along to the figure you need and look at the next digit.
  3. Round, then fill the gaps with zeros so the number keeps its size.

Watch out: Writing 114,729 to 1 s.f. as 1. It is 100,000: keep the place value.

Worked example · Practise this skill free

Negative numbers

  1. Adding a negative is the same as subtracting.
  2. Subtracting a negative is the same as adding.
  3. Multiplying or dividing: same signs give +, different signs give −.

Watch out: Thinking −12 + (−12) is 0. Two negatives added make a bigger negative: −24.

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Ordering numbers

  1. Write every number with the same number of decimal places.
  2. Negatives come first, and the bigger the negative, the smaller it is.
  3. Compare digit by digit from the left.

Watch out: Thinking 9.26 is bigger than 9.5 because 26 is bigger than 5. Compare 9.26 with 9.50.

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Place value

  1. Find the decimal point.
  2. To the left: ones, tens, hundreds, thousands. To the right: tenths, hundredths.
  3. The value is the digit × its place: 4 in the tens place is 40.

Watch out: Confusing tens with tenths. Tenths come after the decimal point.

Worked example · Practise this skill free

Estimating

  1. Round every number to 1 significant figure.
  2. Write the rounded calculation down: that is your method mark.
  3. Work it out without a calculator.

Watch out: Working out the exact answer. An estimate must show the rounded numbers.

Worked example · Practise this skill free

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