Subtraction — Victorian Year 4 Maths
Estimating differences by rounding to the nearest 10 or 100, choosing reasonable answers and checking subtraction by adding back. One subtopic and 20 questions with worked solutions, aligned to VC2M4N07.
Subtraction in Victorian Year 4
This topic is about knowing whether a subtraction answer makes sense. Students round each number to the nearest 10 or 100, subtract the rounded numbers for a quick estimate, and then compare it with the exact answer or check by adding back.
Its one subtopic is Estimate and Check Subtraction; written subtraction of four-digit numbers is taught next to addition in the Addition topic. Subtraction is one of the 20 topics in Victorian Year 4 Maths and belongs to the Number strand. It comes under Victorian Curriculum code VC2M4N07, about using estimation and rounding to check answers. The same checking habits come back in multi-step problems and in Money.
Subtopics
Subtraction: common questions
How do you estimate a difference?
Round each number to the nearest 10 or 100, then subtract the rounded numbers. For \(732 - 289\), round to \(700 - 300 = 400\). The exact answer is \(443\), which is close enough to the estimate to be reasonable.
How do you check a subtraction with addition?
Add the answer to the number you took away. If \(805 - 367 = 438\), then \(438 + 367\) should give back \(805\). If it does not, look for a slip in the trading.
Why is an estimate sometimes higher or lower than the exact answer?
It depends on which way each number was rounded. In \(64 - 47\), \(64\) rounds down and \(47\) rounds up, so \(60 - 50 = 10\) is lower than the exact answer, \(17\). Talking about this helps children judge how close an estimate should be.
What mistakes do children make, and how can they practise at home?
Children sometimes round only one of the numbers, or treat an estimate as the final answer. At home, ask how much change there will be from \(\$50\) for a \(\$32\) item: round it to \(\$30\), estimate about \(\$20\), then work out the exact \(\$18\).
Which curriculum codes cover Subtraction in Victorian Year 4?
The subtopic comes under Victorian Curriculum 2.0 code VC2M4N07, about using estimation and rounding to check whether an answer, including a money calculation, is reasonable.