Decimals and Percentages

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# Decimals and Percentages - PowerPoint PPT Presentation

Decimals and Percentages. Marie Hirst , Numeracy Facilitator, m.hirst@auckland.ac.nz Mathematics Lead Teacher Symposium Waipuna Conference Centre September 2011. To be a proportional thinker you need to be able to think multiplicatively. How do you describe the change from 2 to 10?.

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Decimals and Percentages

Marie Hirst, Numeracy Facilitator,

m.hirst@auckland.ac.nz

Waipuna Conference Centre

September 2011

To be a proportional thinker you need to be able to think multiplicatively

How do you describe the change from 2 to 10?

Views the change as an addition of 8

Multiplicative Thinking:

Views the change as multiplying by 5

Proportional Thinking

A sample of numerical reasoning test questions as used for the NZ Police recruitment

½ is to 0.5 as 1/5 is to

a. 0.15

b. 0.1

c. 0.2

d. 0.5

1.24 is to 0.62 as 0.54 is to

a. 1.08

b. 1.8

c. 0.27

d. 0.48

If a man weighing 80kg increased his weight by 20%, what would his weight be now?

a. 96kg

b. 89kg

c. 88kg

d. 100kg

Developing Proportional thinking

Fewer than half the adult population can be viewed as proportional thinkers

And unfortunately…. We do not acquire the habits and skills of proportional reasoning simply by getting older.

Objectives
• Understand common decimal place value misconceptions and how to address these.
• Develop content knowledge of how to add, subtract and multiply decimals.
• Develop content knowledge of calculating percentages
• Become familiar with useful resources
Decimals

Decimals are special cases of equivalent fractions where the denominator is always a power of ten.

Misconceptions with Decimal Place Value:How do these children view decimals?
• Bernie says that 0.657 is bigger than 0.7

(decimals are 2 separate whole number systems separated by a decimal point, 657 is bigger than 7, so 0.675 is bigger than 0.7)

2. Sam thinks that 0.27 is bigger than 0.395

(the more decimal places, the tinier the number becomes, because thousandths are really small)

3. James thinks that 0 is bigger than 0.5

(decimals are negative numbers)

• Adey thinks that 0.2 is bigger than 0.4

(direct link to fractional numbers , i.e. ½ = 0.2, ¼ = 0.4)

5. Claire thinks that 10 x 4.5 is 4.50

(when you multiply by 10, just add a zero)

Use materials to develop an understanding of decimal tenths and hundredths place value

Use decipipes, candy bars, or decimats to understand how tenths and hundredths arise and what decimal numbers ‘look like’

3 ÷ 5

3 chocolate bars shared between 5 children.

30 tenths ÷ 5 =

0 wholes + 6 tenths each = 0.6

0

6

Connecting the Place Value

5 ÷ 4 = 1 whole + 2 tenths + 5 hundredths

1

2

5

• Understand how tenths and hundredths arise
• express remainders as decimals

BIG IDEA

The CANON law in our place value system is that ONE unit must be split into TEN of the next smallest unit AND NO OTHER!

Using Decipipes: Book 7 p.38-41(Understanding how tenths and hundredths arise)

What is 1 quarter as a decimal?

View children’s response to this task:

Make and compare decimals

• Which is bigger: 0.6 or 0.43?
• How much bigger is it?

• Rank these questions in order of difficulty.
• 0.8 + 0.3,
• 0.6 + 0.23
• 0.06 + 0.23,

Exchanging ten for 1

Mixed decimal place values

Same decimal place values

Add and Subtract decimals (Stage 7)

Place Value

Tidy Numbers

1.5 - 0.9

Reversibility

Standard written form (algorithm)

Add and Subtract decimals (Stage 7)

Place Value

Tidy Numbers

1.6 - 0.98

Reversibility

Standard written form (algorithm)

When you multiply the answer always gets bigger.

True False

0.4 x 0.3Which is the correct answer?0.12 1.2 0.012

Multiplying Decimals by a whole number(Stage 7)

Tidy Numbers

Place Value

5 x 0.8

Convert to a fraction, e.g. x 0.25 = ¼ of

Standard written form (algorithm)

Multiplying a decimal by a decimal (Stage 8)

using Arrays

0.4 x 0.3

0.3

0

1

0.4

Ww

w

1

Using Arrays

0.4 x 0.3 = 0.12

0.3

0

1

0.12

0.4

Ww

w

1

1.3 x 1.4

1

0.4

1

0.3

1.3 x 1.4

1

0.4

= 1.82

1

0.4

1

0.3

0.12

0.3

1.3 x 1.4

0.4

1

1

0.4

1

0.12

0.3

0.3

0.7 x 1.6

1

0.6

= 1.12

0

0.0

0

0.42

0.7

0.7

=

Why calculate percentages?

It is a method of comparing fractions by giving both fractions a common denominator i.e. hundredths.

So it is useful to view percentages as hundredths.

Applying Percentages

Types of Percentage Calculations at Level 4 (stage 7)

• Estimate and find percentages of amounts,
• e.g. 25% of \$80
• Expressing quantities as a percentage
• (Using equivalence)
• e.g. What percent is 18 out of 24?
Estimate and find percentages of whole number amounts.

25% of \$80

Using common conversions halves, thirds, quarters, fifths, tenths

Book 8:21 (MM4-28) , Decimats. Bead strings, slavonic abacus

Practising instant recall of conversions

Bingo, Memory, I have, Who has, Dominoes,

35% of \$80

Using benchmarks like 10%, and ratio tables

FIO: Pondering Percentages NS&AT 3-4.1(p12-13)

10%

\$8

30%

\$24

5%

\$4

\$4

\$8

\$8

\$8

Find 35% of \$80

35%

\$28

Now try this…

46% of \$90

46% of 90

46% of \$90

Is there an easier way to find 46%?

Estimating Percentages

16% of 3961 TVs are found to be faulty at the factory and need repairs before they are sent for sale. About how many sets is that?

(Book 8 p.26 - Number Sense)

Decimal Games and Activities
• First to the Draw
• Four in a Row Decimals
• Beat the Basics
• Decimal Keyboard Games
• Target (Figure It Out)
• Decimal Jigsaw
• Percents
• Decimal Sort

What is this game aimed at?

How could you adapt it to make it easier / harder?

http://teamsolutions.wikispaces.com/

Objectives

• Understand common decimal place value misconceptions and how to address these.
• Develop content knowledge of how to add, subtract and multiply decimals.
• Develop content knowledge of calculating percentages
• Become familiar with useful resources.

What do you know now that you didn’t know before?

What parts of this workshop could you share back with your staff?

Thought for the day

A DECIMAL POINT

When you rearrange the letters becomes

I'M A DOT IN PLACE