# To perform mental calculations

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## Memorandum

1. NUMBER CONCEPT

1.1 27

1.2 79

1.3 96

1.4 63

1.5 42.5

1.6 3.2

1.7 100

1.8 300

1.9 294

1.10 992

1.11 8

1.12 55

1.13 63

1.14 +

1.15 445

## Solve the following puzzle

 B C E H I M N O P R T U 17 11 5 14 21 7 2 6 8 4 18 12

## Clues:

 1. 24 ÷ 12 8. 36 ÷ 6 2. 5 + 7 9. 14 ÷ 7 3. 11 - 4 10. 17 − 6 4. 8 + 9 11. 25 ÷ 5 5. 20 ÷ 4 12. 2 × 4 6. 13 − 9 13. 27 − 9 7. 20 - 9

## Activity: to perform mental calculations [lo 1.9.1, lo 1.9.2]

1. In Mathematics it is important to be able to think fast. Let’s see how quick you are in your first mental arithmetic test in Grade 6. Work as quickly and as accurately as possible.

## 1. think fast!

In Mathematics it is important to be able to think fast. Let’s see how quick you are in your first mental arithmetic test in Grade 6. Work as quickly and as accurately as possible.

 1.1 56 + ................. = 83 1.4 .................  9 = 7 1.2 49 × 15 = ................. 1.5 Halve 85: ................. 1.3 12 × 8 = ................. 1.6 Double 163: ................. 1.7 20 × ................. = 2 000 1.12 A half × ................. = 150 1.8 (7 × 9) − 8 ................. 1.13.. (38 + 49) − 24 = ................. 1.9 98 + 99 + 97 = ................. 1.14 89 ................. 16 = 105 1.10 1 004 − 12 = ................. 1.15 998 − ................. = 550 1.11 8 × 9 × 6 = 6 × ............ × 9

Complete: I have _____________________________________ correct!!

## Did you know?

Can you remember what numbering system we make use of today?

____________________________________________________________________

The ancient Romans developed their counting system more than 2 000 years ago and some of their numbers looked like this: I ; II ; III ; IV ; V

## 2. let’s count

Work with a friend and take turns to count in:

2.1 tens from 19 870 to 20 040

2.2 hundreds from 67 403 backwards to 65 903

• thousands from 37 556 backwards to 29 556
• ten thousands from 25 526 to 95 526

## 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 involving:

1.9.2: multiplication of whole numbers to at least 12 x 12.

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