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Maths · Addition & Subtraction

Mental strategies

Add and subtract two-digit numbers in your head.

Not startedCAMBRIDGE · 3CBSE · IIIIB · PYP-3

Why mental maths matters

Sometimes you need a quick answer without paper or a calculator. Learning to add and subtract in your head helps you think faster. It makes everyday maths much easier and more fun!

Splitting numbers into tens and ones

Every two-digit number has tens and ones. For example, 34 is 30 and 4. When you add or subtract, deal with the tens first. Then handle the ones. This trick makes mental maths much simpler.

Jumping along a number line in your mind

Imagine hopping along a number line. To add 23 to 45, jump forward 20 first. That gets you to 65. Then jump forward 3 more to reach 68. Breaking the number into jumps makes it easier.

Using friendly numbers

Round to a nearby number that is easy to work with. To add 38, round it to 40 first. Then take away the extra 2 at the end. This is called compensation and it saves time.

Subtracting by adding on

Sometimes adding on is easier than taking away. To find 82 minus 56, think: what do I add to 56 to reach 82? Count up: 4 gets you to 60, then 22 more reaches 82. So 4 plus 22 equals 26.

Worked examples

  1. 1. 47 + 25

    1. Split 25 into 20 and 5.
    2. Add the tens first: 47 + 20 = 67.
    3. Now add the ones: 67 + 5 = 72.
    4. Answer: 72

  2. 2. 63 - 28

    1. Round 28 up to 30 to make it easier.
    2. Subtract 30 from 63: 63 - 30 = 33.
    3. You subtracted 2 too many, so add 2 back: 33 + 2 = 35.
    4. Answer: 35

  3. 3. 54 + 39

    1. Round 39 up to 40.
    2. Add 40 to 54: 54 + 40 = 94.
    3. You added 1 too many, so subtract 1: 94 - 1 = 93.
    4. Answer: 93

Try it yourself

Find 71 - 34 using mental maths.

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