The idea in brief. Multiply one place at a time, ones first, and when a place's product is ten or more, rename it: write the ones digit and carry the tens into the next place. Multiplying by a 2-digit number is two of these, the second one shifted one place left because it is multiplying by tens, and then an addition.
Lesson video, with the working
Multiplying by one or two digits
4 min 44 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
How can I help my child understand written P4 multiplication?
Your child extends written multiplication to the two sizes named in the MOE Primary 4 syllabus: up to 4 digits by 1 digit and up to 3 digits by 2 digits.
What your child learns here
Your child extends written multiplication to the two sizes named in the MOE Primary 4 syllabus: up to 4 digits by 1 digit and up to 3 digits by 2 digits. Place value explains each stage, including renaming and multiplying by tens. The aim is a method your child can account for line by line.
Connect multiplication with equal groups, identifying the amount in each group and the number of groups before using a written algorithm.
Multiply through the places of a multi-digit number and rename ten of one place into the next when a partial result requires it.
Treat the tens digit of a 2-digit multiplier as a number of tens, so that its partial product has the correct place value.
Add the partial products to obtain the full product, maintaining the alignment of ones, tens, hundreds and thousands throughout the calculation.
Use a nearby easy multiplication to judge the size of the product, while keeping the exact working for the final answer.
Does the answer make sense?
When multiplying a positive number by a 2-digit number between 20 and 30, the product must lie between that number multiplied by 20 and by 30. Your child can calculate those two bounds to detect a product that is far too large or too small.
How to practise these questions
Basic practice finds a total cost from equal-priced items. Intermediate questions include multiplying by a multiple of ten, finding totals from equal cartons or repeated production, handling a zero within a multiplicand and explaining why a proposed product is too small. Challenge questions recover a missing multiplier or combine two orders of chairs before finding the total cost. Ask your child to identify the value represented by the multiplier before calculating each partial product. For the reasonableness question, an easy lower bound can support the explanation as well as the exact multiplication. After comparing with the written working, return to the particular place where renaming or multiplying by tens changed the result, rather than repeating unrelated calculations.
Questions parents ask
What multiplication sizes are required in Primary 4? The MOE outcome specifies up to 4 digits multiplied by 1 digit, and up to 3 digits multiplied by 2 digits. These are two different cases. Your child can practise each within its stated range, building on the multiplication facts and place value learned in earlier years.
Why is the second partial product different when multiplying by tens? That part of the calculation represents groups of ten rather than groups of one. Ask your child to say the value of the multiplier's tens digit before calculating. The written placement then follows from that value, giving a reason for the method beyond remembering a layout.
Should we practise the tables separately from long multiplication? If recalling a multiplication fact interrupts every step, a short review of that fact can help. Your child also needs to explain renaming and addition of partial products, so table recall alone is not the whole skill. Identify whether the hesitation concerns a fact or a place-value decision.
