1A
3 Multiplication and division (1)
Quick reference
Multiplying and dividing by 10, 100 and 1000
Core activity 3.1: Multiply and divide by 10, 100 and 1000 (Learner’s Book p11) Learners multiply and divide whole numbers by 10 and 100, extending to multiplying and dividing by 1000.
I multiply my number by 100, then divide by 10, then multiply by 1000. My answer is one hundred and seventy thousand.
1 1
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! 10 " 2500
What is the missing number?
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A decagon has 10 sides.
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2 What is the perimeter of a regular decagon with sides 17 centimetres long? 4
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Use the given fact to derive a new fact and then explain your method. Fact
# 100 " 250
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100 ! 10 " 10 000 #
Core activity 3.2: Mental strategies for multiplication (Learner’s Book p13) Learners develop and refine mental strategies for multiplication, including working with multiples and near multiples of 10, halving and doubling.
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250 # 10 "
Let’s investigate
Find different ways of completing this calculation.
Copy and complete the table, the first one has been done for you.
Copy and complete this set of missing numbers. 25 ! 100 "
Mental strategies for multiplication
Vocabulary near multiple of 10: a number either side of a multiple of 10. For example, 20 is a multiple of 10, so 19 and 21 are near multiples of 10.
Let’s investigate
Cheng is thinking of a number. What number is Cheng thinking of?
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Milly says, “Every multiple of 1000 is divisible by 100.” Is she right?
Derived fact
a
7 ! 9 " 63
7 ! 18 " 126
b
7 ! 3 " 21
70 ! 3 "
c
5 ! 7 " 35
50 ! 70 "
d
6 ! 8 " 48
6 ! 16 "
e
8 ! 13 " 104
4 ! 13 "
f
6 ! 7 " 42
6 ! 70 "
g
5 ! 9 " 45
5 ! 91 "
6 ! 9 " 54
6 ! 89 "
h i
4 ! 7 " 28
j
3 ! 9 " 27
Method 18 is double 9 so double the answer
39 ! 7 " 30 ! 91 "
Use table facts to help you work out the following: (a) 30 × 70
(b) 50 × 9
(d) 50 × 80
(e) 8 × 90
(c) 20 × 6 (f) 70 × 60
Work out the following using a mental strategy: (a) 29 × 6
(b) 41 × 5
(c) 19 × 7
(d) 21 × 8
(e) 49 × 6
(f) 51 × 4
Explain to your partner how you worked out the answers.
Explain your answer. For more questions, turn the page ... Unit 1A: Core activity 3.1 Multiply and divide by 10, 100 and 1000
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Unit 1A: Core activity 3.2 Mental strategies for multiplication
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please note that listed objectives might only be partially covered within any given chapter but are covered fully across the book when taken as a whole
Prior learning
Objectives* –
This chapter revises work done in Stage 5 when learners multiplied and divided whole numbers by 10 and 100, and worked on mental strategies for multiplication.
1A: Numbers and the number system 6Nn4 – Multiply and divide any whole number from 1 to 10 000 by 10, 100 or 1000; explain the effect. 1A: Calculation (Multiplication and division) 6Nc8/6Nc14 – Multiply pairs of multiples of 10, e.g. 30 × 40, or multiples of 10 and 100, e.g. 600 × 40. 6Nc15 – Multiply near multiples of ten by multiplying the multiple of ten and adjusting. 6Nc16 – Multiply by halving one number and doubling the other, e.g. calculate 35 × 16 with 70 × 8. 1A: Problem solving (Using techniques and skills in solving mathematical problems) 6Pt1 – Choose appropriate and efficient mental or written strategies to carry out a calculation involving addition, subtraction, multiplication or division. 1A: Problem solving (Using understanding and strategies in solving problems) 6Ps1 – Explain why they choose a particular method to perform a calculation and show working. 6Ps4 – Use ordered lists or tables to help solve number problems systematically. 6Ps6 – Make sense of and solve word problems and represent them.
*for NRICH activities mapped to the Cambridge Primary objectives, please visit www.cie.org.uk/cambridgeprimarymaths
Vocabulary near multiple of 10
Cambridge Primary Mathematics 6 © Cambridge University Press 2014
Unit 1A
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