# GCSE Maths Starter 18 - PowerPoint PPT Presentation

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GCSE Maths Starter 18. 4. Write these numbers in order of size, smallest first: 1.8, 3.71, 0.5, 12.4 5. Write down the reciprocal of ¼ A small pot costs 20 pence. A large pot costs 150% more, how much does the large pot cost ?

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GCSE Maths Starter 18

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### GCSE Maths Starter 18

4. Write these numbers in order of size, smallest first:

1.8, 3.71, 0.5, 12.4

5.

• Write down the reciprocal of ¼

• A small pot costs 20 pence. A large pot costs 150% more, how much does the large pot cost?

• Copy the pattern into your book, add one more square so the pattern has one line of symmetry

## Lesson 18 Estimation and using a calculatorMathswatch clip (14/101/63).

To round numbers to a given degree of accuracy (Grade E )

EXTN:

To calculate an estimate for a given sum (Grade C)

Significant Figures (Rounding)

Numbers can be rounded to 1,2, 3 or more significant figures. We count the number of figures from the firstnon-zero digit.

First non-zero digit.

Rounding to 1 s.f

5 or bigger ?

5 or bigger ?

5 or bigger ?

4

6

0.04

4 . 3 3 2 5

0. 0 4 2 5

5. 7 4 2 5

No

No

Yes

First non-zero digit.

First non-zero digit.

5 or bigger ?

5 or bigger ?

0.08

0.002

Significant Figures (Rounding)

Rounding to 1 s.f

0. 0 0 1 5

0 . 0 7 6

Yes

Yes

First non-zero digit.

5 or bigger ?

5 or bigger ?

1.5

0.054

Significant Figures (Rounding)

Rounding to 2 s.f

1 . 4 7 2 9

0. 0 5 3 5

Yes

Yes

First non-zero digit.

5 or bigger ?

5 or bigger ?

0.0205

2.08

Significant Figures (Rounding)

Rounding to 3 s.f

2 . 0 7 5 9

0. 0 2 0 4 6 3

Yes

Yes

Lesson 18 Estimation and using a calculatorMathswatch clip (14/101/63).

The following number could be the population of a country at a particular instant in time.Write this number to 1, 2, 3, 4, 5, 6 and 7 significant figures.

56 345 678

56 000 000

56 300 000

60 000 000

56 345 700

56 350 000

56 346 000

56 345 680

### Estimation

“is approximately equal to”

Martin uses his calculator to work out 39 × 72.

The display shows an answer of 1053.

How do you know this answer must be wrong?

39 × 72 

40 × 70 =

2800

Also, if we multiply together the last digits of 39 and 72 we have

9 × 2 = 18.

9 × 2 = 18.

The product of 39 and 72 must therefore end in an 8.

### Estimation

How could we estimate the answer to 3.5 × 17.5?

3.5 × 17.5 can be approximated to:

4 × 20 =

80

3 × 18 =

54

4 × 17 =

68

or between 3 × 17 =

51

and 4 × 18 =

72

### Estimation

How could we estimate the answer to 4948 ÷ 58?

4948 ÷ 58 can be approximated to:

5000 ÷ 60 =

?

(60 does not divide into 5000)

5000 ÷ 50 =

100

4950 ÷ 50 =

99

or 4800 ÷ 60 =

80

Lesson 18 Estimation and using a calculatorMathswatch clip (14/101/63).

Estimate the following. (Clearly show your rounded values)

1500

20,000

8,000,000

1)29 × 512)431 × 483)2184 × 3962

4)17.41 × 8.735)4.372 × 1.8216)5.32 × 0.236

7)0.731 × 0.4898)0.0813 × 0.279)1.043 × 4.21

10)2.73 × 4.1 × 6.211)2.43 × 0.045 × 312)0.23 × 2.74 × 3.05 × 0.97

180

8

1

0.35

0.024

4

0.3

72

1.8

### Solving complex calculations mentally

3.2 + 6.8

What is

?

7.4 – 2.4

3.2 + 6.8

10

=

=

7.4 – 2.4

5

2

We could also write this calculation as: (3.2 + 6.8) ÷ (7.4 – 2.4).

How could we work this out using a calculator?

### Using bracket keys on the calculator

3.7 + 2.1

What is

?

3.7 – 2.1

3.7 + 2.1

6

=

3.7 – 2.1

2

We start by estimating the answer:

3

Using brackets we key in:

(3.7 + 2.1) ÷ (3.7 – 2.1) =

3.625

9 + 30

Four people used their calculators to work out .

15 – 7

## Lesson 18 Estimation and using a calculatorMathswatch clip (14/101/63).

Some for you to try..

Who is correct?

What did the others do wrong?

## Lesson 18 Estimation and using a calculatorMathswatch clip (14/101/63).

Some for you to try..

Lesson 18 Estimation and using a calculatorMathswatch clip (14/101/63).

Exam questions