The idea in brief. Multiply as if the point were not there, one place at a time, renaming whenever a place makes ten or more. Then put the point in the product directly under the point in the number you multiplied: the multiplier is a whole number, so it cannot move the point.
Lesson video, with the working
Multiplying a decimal by a one-digit number
3 min 22 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
How can I explain multiplying decimals by a whole number to my child?
Your child multiplies a decimal amount by a one-digit whole number.
What your child learns here
Your child multiplies a decimal amount by a one-digit whole number. In the MOE P4 syllabus, the decimal has up to two decimal places. Equal groups give the calculation its meaning: each group contains the same amount, and multiplication finds the combined value while preserving the units named in the question.
Interpret the whole-number multiplier as the number of equal groups, connecting a multiplication calculation with repeated addition of the decimal amount.
Multiply hundredths, tenths and ones using place value, regrouping whenever ten smaller parts form one part of the next size.
Keep the decimal point consistent with what the digits count, so a whole-number multiplication fact supports the decimal calculation without changing its meaning.
Use multiplication to find a total cost, length or other amount when the number of identical units and the amount for one unit are known.
Estimate from nearby whole numbers and check the unit of the product before deciding whether the calculated result fits the situation.
Does the answer make sense?
Since 1.84 lies between 1 and 2, four equal groups of 1.84 must total more than 4 and less than 8. Your child can use those bounds to reject a misplaced decimal point before checking the exact multiplication.
How to practise these questions
The Basic question finds a total volume from identical bottles. Intermediate questions combine a repeated amount with a different amount, calculate change after purchases, or find a length remaining after equal pieces are cut. The Challenge questions add another grouping step for filled containers or compare two item prices before calculating change. Ask your child to say what one group contains and how many such groups are needed before multiplying. When an extra amount is involved, establish whether it belongs to every group or appears just once. After the product is found, reread the question to decide whether that product is the final answer or an amount needed for another step.
Questions parents ask
Does P4 decimal multiplication include multiplying two decimals? The Primary 4 outcome on this page is multiplication of a decimal, up to two decimal places, by a one-digit whole number. Both factors being decimals is a different calculation. Keep the group size and the whole-number count clear when helping your child interpret these questions.
How can multiplication facts help with a decimal product? The same facts apply to different-sized parts. If your child knows how many groups of a whole number make a product, they can use that fact while tracking whether the digits now count tenths or hundredths. Saying the place-value unit gives a reason for the decimal point's position.
Should multiplication always make a number larger? For a positive decimal multiplied by a whole number greater than one, the product is larger than that decimal. Multiplying it by one leaves it unchanged, and multiplying it by zero gives zero. Ask your child to look at the multiplier before using size as a check.
