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SATs practice · Year 6Chapter 7 · Mental arithmetic
Year 6 Arithmetic (SATs) · Mental arithmetic · Lesson 7.3

Year 6 Arithmetic (SATs): Doubling, halving and multiplying by 10 and 100

Part of the Year 6 Arithmetic (SATs) mental arithmetic chapter. Chapter overview · whole chapter on one page

7.3

Doubling, halving and multiplying by 10 and 100

Year 6
Multiply and divide mentally, drawing on known facts, including doubling and halving (Year 5)
Story. A baker makes 3,450 buns on Monday. On Tuesday she makes double that number. She wants to know how many buns she made on Tuesday.

Rule

Doubling means multiply by 2; halving means divide by 2. Double the number, or halve it, one part at a time.

Multiplying by 10 moves digits one place left; by 100, two places left.

Key ideas

  • Double: add the number to itself, or double each part.
  • Halve: halve each part, then add the halves.
  • Multiply by 100 by adding two zeros.
Worked example 7.3.1
Double 3,450.

Method. Double each part of the number.

  1. Split the number. 3,450 is 3,000 + 400 + 50.
    3,000 + 400 + 50
  2. Double each part. 6,000 + 800 + 100.
    6,000 + 800 + 100
  3. Add. 6,000 + 800 + 100 = 6,900.
    6,900
  4. Write the answer.
    6,900

Check. 6,900 ÷ 2 = 3,450. ✓

Worked example 7.3.2
Halve 764.

Method. Halve each part of the number.

  1. Split the number. 764 is 700 + 60 + 4.
    700 + 60 + 4
  2. Halve each part. 350 + 30 + 2.
    350 + 30 + 2
  3. Add. 350 + 30 + 2 = 382.
    382
  4. Write the answer.
    382

Check. 382 × 2 = 764. ✓

Worked example 7.3.3
Work out 46 × 100.

Method. Multiply by 100 by moving the digits two places left.

  1. Move the digits. 46 moves two places left.
    46 → 4,600
  2. Check. 4,600 ÷ 100 = 46.
    4,600
  3. Write the answer.
    4,600

Check. 46 with two zeros is 4,600. ✓

Exercise 7C  Practice: doubling, halving and multiplying by 10 and 100

Non-calculator
1
FDouble 2,450.
2
FDouble 6,780.
3
FDouble 34,500.
4
FDouble 1,250.
5
FDouble 480.
6
FDouble 9,900.
7
FHalve 1,840.
8
FHalve 6,300.
9
FHalve 25,000.
10
FHalve 740.
11
FHalve 9,700.
12
FWork out 37 × 100.
13
FWork out 540 × 10.
14
PSTom has 3,450 stickers and his sister has double that. How many stickers does his sister have?
15
PSA bus carries 480 people in a day. How many people does it carry in 10 days?
16
PSA box holds 25 pencils. How many pencils are in 100 boxes?
17
RHalve 1,000 and then halve the answer. What is the result?
18
RExplain why doubling 2,450 and then halving the answer gives 2,450 again.

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