Aligning decimals

Having been taught that the units must be lined up in column addition or subtraction, a common misconception is that the right hand side of a calculation must always be aligned. Whilst true for whole numbers, this is not always the case when decimals are involved and pupils can find it difficult to accept that calculations involving decimals can look quite uneven when set out.

23.29 + 65.4

For example, when setting out 23.29 + 65.4, pupils may incorrectly attempt to align the digits on the right hand side.

23.29 + 65.4

The pupil must understand that the 5 units in the bottom number must be aligned directly beneath the 3 units in the top number and that the easiest way to do this is to ensure that the decimal points are aligned.

23.29 + 65.4

Sometimes, pupils may insert a zero to make both numbers line up and may need support to recognise that this alters the value of the 4 in the number.

If a pupil is just told to line up the tens in this calculation, this can lead to a further misconception that the left side must always align instead! So setting out the calculation 26.703 + 300.8 may then result in:

instead of

Reinforcement of decimal place value may be necessary to improve understanding and confidence in setting out such calculations.

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