Arithmetic Tricks Flashcards Preview

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Flashcards in Arithmetic Tricks Deck (55):
1

Find the multiple of two numbers separated by 2 (e.g., 12 and 14)

square the number in the middle and subtract 1 (e.g., 13 x 13 = 169-1 = 168)

2

225

15 squared

3

121

11 squared

4

400

20 squared

5

625

25 squared

6

169

13 squared

7

900

30 squared

8

196

14 squared

9

125

5 cubed

10

8

2 cubed

11

216

6 cubed

12

27

3 cubed

13

1000

10 cubed

14

64

4 cubed

15

4ˆ4

256

16

3ˆ3

27

17

4ˆ3

64

18

5ˆ3

125

19

2ˆ4

16

20

3ˆ4

81

21

4ˆ4

256

22

5ˆ4

625

23

2ˆ5

32

24

3ˆ5

243

25

2ˆ6

64

26

Square root of 1

1

27

Square root of 2

1.4

28

Square root of 3

1.7

29

Square root of 5

2.2

30

Square root of 6

2.4

31

Square root of 7

2.6

32

Square root of 8

2.8

33

Square root of 10

3.2

34

> sq. root 4 and < sq. root 10

Add 0.2 (sq root 4=2, sq root 5 = 2.2, sq. root 6 = 2.4, etc.)

35

< sq. root 4

Subtract 0.3 (sq. root 4 = 2, sq. root = 1.7, etc.)

36

1/8 =

0.125

37

1/11

0.09090909090909

38

1/6=

0.16161616161616...

39

1/9=

0.111111

40

1/99=

0.0101010101010101

41

1/4=

0.25

42

1/7=

Aprox. 0.14

43

To multiply or divide a number by 4

DOUBLE or HALVE it 2x

44

To multiply or divide a number by 8

DOUBLE or HALVE it 3x

45

Multiplying by 9

MULTIPLY by 10, then subtract original number (23 x 9 = 23*10 - 23 = 230 - 23 = 207

46

Multiplying by 11

Multiply by 10 and ADD the original number number (45 x 11 = 45 x 10 + 45 = 450 + 45 = 495)

47

Multiplying and dividing by 5

Multiply the number by 10, then divide the answer by 2

(23*5 = (23*10)/2 = 230/2 = 115)
OR

Divide the number by 10, then multiply by 2
(96/5 = (96/10) x 2 = 9.6 x2 = 19.2)

48

Subtraction by addition ! GO

Number line
OR
Guess (X + WHAT NUMBER gets me close to Y?)

49

To easily get 15% of a number

First get 10% (slide decimal point one spot to left, 10% of 100 is 10, 10% of 53 is 5.3)

Then get 5% by taking half of 10% (5% of 53 = 2.6ish)

Then add (5.3 + 2.6 = 7.9 = 15% of 53)

50

To get 1% of a number

Slide the decimal point TWO spots to the left

(1% of 34 = 0.34, 1% of 0.07 = 0.0007)

51

Divisibility rules for 8

Last 3 digits can be cut in half 3x

52

Divisibility rules for 3

Sum if its digits is divisible by 3

53

Divisibility rules for 9

Sum of its digits is divisible by 9

54

Divisibility rules for 6

If divisible by 2 and 3

55

Divisibility rules for 4

If it's last 2 digits can be cut in half 2x