The idea in brief. Split a number into useful parts, or use a convenient number and undo the extra change. If you add or subtract extra to make a convenient number, undo that extra change. In the lesson. In Primary 2, you added and subtracted tens mentally. Today, we add and subtract mentally by using an easy number close by. Use an easy number close by, then undo the extra.
Lesson video, with the working
Adding and subtracting mentally
3 min 44 s
The film teaches 3 questions in turn. Each question shows first; its working is written as the film explains it.
How do P3 children solve mental addition and subtraction questions?
This page teaches mental addition and subtraction of two 2-digit numbers, the MOE Primary 3 outcome on mental calculation.
What your child learns here
This page teaches mental addition and subtraction of two 2-digit numbers, the MOE Primary 3 outcome on mental calculation. The main strategy is to use an easy number close by, such as 40 for 39, and then adjust.
Add or subtract a 2-digit number in parts, tens first and then ones: 63 plus 25 is 63 plus 20, then plus 5, which is 88.
Use a nearby tens number to make the work easier: 46 plus 29 is 46 plus 30, minus 1, which is 75.
Know which way to adjust: after adding too much, subtract the extra; after subtracting too much, add it back.
Choose between splitting and using a nearby number, depending on the numbers in the question.
Explain the steps in words, so someone else can follow and check the method.
Does the answer make sense?
Two 2-digit numbers added together give less than 200, and subtracting one from another gives less than the first number. Your child can also glance at the ones digits: 9 plus 6 must end in 5, so 59 plus 36 has to end in 5.
How to practise these questions
The Basic questions practise splitting numbers into tens and ones to add or subtract in the head. The Intermediate questions add crossing a ten, where the ones part needs adjusting. The Challenge questions call for a quicker route, such as rounding to a near ten and then adjusting the answer. When a mental sum is stuck, play its solution film together, then have another go later from Revision.
Questions parents ask
What mental calculation is in the Primary 3 syllabus? The MOE Primary 3 syllabus lists mental calculation involving addition and subtraction of two 2-digit numbers. This page covers that outcome, including sums that go past 100.
Is it fine if my child uses a different mental method? Yes, as long as each step is correct and your child can explain it. This page teaches splitting into tens and ones and using an easy number close by; some children prefer one, some the other.
Should mental sums be written down? The working stays in the head, but writing the answer and, at first, one line on how it was found lets you see the method. Once the method is steady, that line can go.
