The idea in brief. When a quantity is shared into the WRONG number of equal pieces, the extra that each piece carries is not waste, the extras, added up, are exactly the piece that was never cut. So the way back is to gather the extras into one missing piece, and count the whole in those. Undo the wrong operation with its opposite: a wrong multiplication is undone by dividing, a wrong addition by subtracting. In the lesson. In the last lesson, we crossed out the items that two purchases both had. Today, we will fix a wrong calculation by undoing it first. First, undo the wrong calculation to find the number. Then do the right calculation.
Lesson video, with the working
Undo the wrong operation
4 min 30 s
The film teaches 3 questions in turn. Each question shows first; its working is written as the film explains it.
Question 1 of 12 · Basic · free to tryA pupil was meant to multiply a number by 6. Instead, the pupil multiplied…A pupil was meant to multiply a…
A pupil was meant to multiply a number by 6. Instead, the pupil multiplied it by 2 and obtained 48. What should the correct result be?
Show the working and the answer
Working
Original number = 48 ÷ 2 = 24
Correct result = 24 × 6 = 144
Answer: 144
For parents
How do you find the correct answer in P3 wrong operation word problems?
In these questions someone was asked to do one calculation but did another, and the wrong result is given.
What your child learns here
In these questions someone was asked to do one calculation but did another, and the wrong result is given. The page applies Primary 3 multiplication and division within the tables, and the idea that division undoes multiplication, to a working-backwards word problem.
Separate what was meant to happen from what did happen.
Undo the operation that was used: divide to undo multiplying, and multiply to undo dividing.
Check the starting number by redoing the wrong calculation.
Apply the intended calculation to the starting number.
Does the answer make sense?
Your child can compare the two operations: if the intended one makes a number bigger than the one used, the correct result must be bigger than the result the pupil got, and smaller if not. So dividing by 8 instead of multiplying by 4 must give far less than 960.
How to practise these questions
The Basic questions practise undoing the operation that was used to recover the starting number. The Intermediate questions add mix-ups where the number used is close to the intended one, or a division was done in place of a multiplication. Some of the Challenge questions swap a multiplication for a division, so the result is undone with the opposite operation before the intended one is applied. If an undoing question stalls, play its solution film and come back to it later from Revision.
Questions parents ask
What is working backwards in P3 maths? It means starting from a result and undoing each operation to find the starting number. Division undoes multiplication and multiplication undoes division, which the P3 work on the tables supports.
Why can't my child do the correct calculation on the wrong answer? The wrong answer is not the starting number; it already has the wrong operation inside it. The starting number has to be recovered first, and only then can the intended calculation be done.
How do I explain inverse operations to my P3 child? Use a small example both ways: 4 groups of 5 make 20, and 20 shared into 4 groups gives 5 back. Ask your child to undo a few of your calculations aloud before trying a word problem.
