# 3.5 Multiplication of decimal fractions

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## Memorandum

20.1

a) 260

b) 2 600

c) 26,7

d) 267

e) 2 670

f) 26,79

g) 267,9

h) 2 679

20.2 x10: Each digit moves 1 place left / comma moves 1 to the right.

x100: Each digit moves 2 places left / comma moves 2 to the right.

x1000: Each digit moves 3 places left / comma moves 3 to the right.

20.4

 a) 54,8 548 5480 b) 3,67 36,7 367 c) 8,9 89 890 d) 165,38 1653,8 16 538 e) 282,51 2825,1 28 251

20.5.1

a) 21,522

b) 46,215

c) 1,4

d) 4,64

e) 12 250

f) 2 370,6

x with number<1

20.8

a) 0,4964

b) 1,3737

c) 0,109

d) 5,53724

e) 38,57997

f) 16,78164

23.1  10: Comma moves 1 place left

Each digit moves 1 place right

 100: Comma moves 2 place left

Each digit moves 2 place right

 1000: Comma moves 3 places left

Each digit moves 3 places right

23.3

 8,21 0,729 0,6 0,864 0,6254 0,0392 8,791 0,035 0,08 74,68

## Activity: multiplication of decimal fractions [lo 1.3.4, lo 1.7.7, lo 1.9.2, lo 1.10, lo 2.3.3]

20. MULTIPLICATION OF DECIMAL FRACTIONS

20.1 Let us revise

Do the following with the help of your calculator:

 SUM ANSWER e.g. 2,6 x 10 26 a) 2,6 x 100 ......................... b) 2,6 x 1 000 ......................... c) 2,67 x 10 ......................... d) 2,67 x 100 ......................... e) 2,67 x 1 000 ......................... f) 2,679 x 10 ......................... g) 2,679 x 100 ......................... h) 2,679 x 1 000 .........................

20.2 Work in groups of three. Quickly formulate rules for the multiplication of decimal fractions by 10, 100 and 1 000.

20.3 Do you still remember?

10 x 1 = 10 = 101

10 x 10 = 100 = 102

10 x 10 x 10 = 1 000 = 103

10 x 10 x 10 x 10 = 10 000 = 104

10 x 10 x 10 x 10 x 10 = 100 000 = 105

10 x 10 x 10 x 10 x 10 x 10 = 1 000 000 = 106

20.4 Complete the following table:

 × 10 1 10 2 10 3 e.g. 3,2 32 320 3 200 a) 5,48 ..................... ..................... ..................... b) 0,367 ..................... ..................... ..................... c) 0,89 ..................... ..................... ..................... d) 16,538 ..................... ..................... ..................... e) 28,251 ..................... ..................... .....................

20.5 More revision

20.5.1 Work with a friend. Calculate the answers without using a calculator:

a) s = 3,587 x 6

………………………………………………

………………………………………………

………………………………………………

………………………………………………

b) f = 9,243 x 5

………………………………………………

………………………………………………

………………………………………………

………………………………………………

c) p = 0,02 x 70

………………………………………………

………………………………………………

………………………………………………

………………………………………………

d) r = 0,058 x 80

………………………………………………

………………………………………………

………………………………………………

………………………………………………

e) t = 24,5 x 500

………………………………………………

………………………………………………

………………………………………………

………………………………………………

f) h = 2,634 x 900

………………………………………………

………………………………………………

………………………………………………

………………………………………………

20.6 MULTIPLYING DECIMAL FRACTIONS WITH DECIMAL FRACTIONS

20.6.1 Read the following problem and work through the different solutions with a friend:

A tortoise walks 0,8 km in 1 hour. How far will he walk in 0,6 of an hour?

a)

I convert the decimal fractions to proper fractions:

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