# 1.23 To be able to do mental arithmetic

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2. Subtraction

3.1 8

3.2 28

3.3 47

3.4 40

3.5 340

3.6 3 580

3.7 18 200

3.8 18 050

3.9 10 250

3.10 8 250

3.11 18

3.12 14

3.13 8

3.14 12

3.15 45

3.16 450

3.17 350

• 25 000

3.19 1 500

3.20 250

Leaner Section

## Activity: to be able to do mental arithmetic [lo 1.9]

1. It is often necessary for us to be able to add or subtract quickly. We must always try to find "shortcuts" for mental arithmetic. Can you think of a quick way of subtracting 9 from a number, for instance? (See who in your class has got the most effective method for this!)

2. Now are you ready for action? Subtract 9 from the following numbers. Find the matching answer in the wheel and write the letter that is paired with it in the space below, to show what this learning unit covers!

 18 31 22 47 25 34 24 47 43 26 14

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

• Do you still recognise these?

In 468 – 231 = 237 we refer to

468 as the minuend

231 as the subtrahend

237 as the difference

 Thus: 468 - 231 = 237 minuend subtrahend difference
• Do you also remember?

The INVERSE (reverse calculation) of subtraction is ADDITION.

If you therefore want to check a subtraction sum, you can add.

3. Let’s practise your skills in mental arithmetic again! Subtract the following as quickly as possible. If you know your place values (refer to Learning Unit 1) it will be child’s play.

2.1 17 – 9 = _________________

2.2 37 – 9 = _________________

2.3 97 – 50 = _________________

2.4 97 – 57 = _________________

2.5 360 – 20 = _________________

2.6 3 600 – 20 = _________________

2.7 18 250 – 50 = _________________

2.8 18 250 – 200 = _________________

2.9 18 250 – 8 000 = _________________

2.10 18 250 – 10 000 = _________________

2.11 _________________ – 9 = 9

2.12 _________________ – 8 = 6

2.13 21 – _________________ = 13

2.14 42 – _________________ = 30

2.15 90 – _________________ = 45

2.16 900 – _________________ = 450

2.17 900 – _________________ = 550

2.18 _________________ – 6 000 = 19 000

2.19 21 500 – _________________ = 20 000

2.20 25 750 – _________________ = 25 500

• Complete by ticking in the applicable column.
 NOT AT ALL ONLY A LITTLE A LOT VERY MUCH I struggle to subtract quickly and accurately I struggle to subtract quickly I struggle to subtract correctly

## Assessment

Learning Outcome 1: The learner will be able to recognise, describe and represent numbers and their relationships, and to count, estimate, calculate and check with competence and confidence in solving problems.

Assessment Standard 1.9: We know this when the learner performs mental calculations.

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