The idea in brief. Line up the decimal points, not the last digits, and fill every empty place with a nought before you start. Once 5.6 is written 5.60 the columns are tenths under tenths and hundredths under hundredths, and the adding or subtracting is the whole-number algorithm the child already runs.
Lesson video, with the working
Adding and subtracting decimals
2 min 59 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
Question 1 of 6 · Intermediate · free to tryMrs Rani had $30. She bought a storybook for $12.45 and a pen for $6.80. How…Mrs Rani had $30. She bought a…
Mrs Rani had $30. She bought a storybook for $12.45 and a pen for $6.80. How much money did she have left?
total cost of the storybook and the pen = 12.45 + 6.80 = $19.25
The storybook and the pen cost $19.25 altogether.
money left = 30 − 19.25 = $10.75
Mrs Rani had $10.75 left.
Answer: $10.75
For parents
How can my child keep place values aligned when adding and subtracting decimals?
Your child adds and subtracts decimals by combining or comparing like place values.
What your child learns here
Your child adds and subtracts decimals by combining or comparing like place values. The MOE Primary 4 outcome covers decimals up to two decimal places. Whole-number column methods still apply, but the decimal point fixes where the ones, tenths and hundredths belong, including when the two numbers have different numbers of written places.
Match ones with ones, tenths with tenths and hundredths with hundredths before carrying out an addition or subtraction calculation.
Write equivalent decimals with trailing zeros where helpful, preserving the amount while making every place in the calculation explicit.
Regroup ten hundredths as one tenth when adding, or rename a larger place into smaller parts when the subtraction requires it.
Choose addition for a combined amount and subtraction for a difference or remainder, using the relationship in the story rather than a single keyword.
Find a missing amount in a number sentence and check it by putting the result back into the original addition or subtraction relationship.
Does the answer make sense?
For 5.20 minus 2.85, a result of 2.35 can be checked by adding 2.35 and 2.85 to recover 5.20. Your child should also expect a positive result smaller than 5.20 because a smaller positive amount was subtracted from it.
How to practise these questions
The questions here are Intermediate and Challenge. Intermediate practice includes money left after purchases, missing amounts in number sentences, and finding a combined capacity after working out which container holds more. The Challenge questions ask your child to work backwards through changes in money or arrange digits to obtain the smallest possible decimal difference. Before any column calculation, ask what the missing amount represents and whether it must be found by addition or subtraction. In a backwards problem, reverse the changes in their proper order. For the digit question, compare the candidate differences and check that each proposed pair uses the permitted digits exactly as required by the question.
Questions parents ask
Why should my child line up decimal points rather than the last digits? The point marks the boundary between ones and tenths, so aligning it also aligns the place values. Aligning only the final digits could put a tenth underneath a hundredth. Ask your child to name the places in one column before calculating to confirm they represent equal-sized parts.
Can a whole number be written with decimal zeros for subtraction? Yes. Writing 6 as 6.00 preserves its value and makes tenths and hundredths available for the column working. When regrouping, the amount remains six wholes overall; some of it is simply renamed into smaller parts so the required subtraction can be completed.
How do I help when my child chooses the wrong operation in a story? Ask which amount is already known and which amount the question wants. A story may describe an increase but ask for the starting amount, so the word describing the action is not enough. Have your child say the relationship between the amounts before choosing addition or subtraction.
