Decimal calculations - Wales Essential Skills

[Pages:12]Application of Number Level 1

Decimal calculations

Introduction

Calculating with money or metric measurements often involves working with decimals. You can use a calculator, but there will be times when you don't have one ? and you can't use a calculator in tests and exams ? so you need to know how to do pen and paper calculations.

This activity looks at how to calculate with decimals. You will learn how to:

n add and subtract decimals n multiply a decimal by a whole number n divide a decimal by a whole number.

You will need a pen and paper for calculations.

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Application of Number Level 1

Decimal calculations Add and subtract decimals

When you add and subtract decimals, write the numbers in columns and line up the decimal points. Make sure you keep each digit in the correct column.

Fill empty columns after the decimal point with zeros. To check your answer, round to whole numbers and do an approximate calculation.

Example 1: 2.75 + 15.9 + 6 2.75

+ 15.90 + 6.00

24.65

Check: 3 + 16 + 6 = 25

Example 2: 12.9 ? 3.75 12.90 ? 3.75 9.15

Check: 13 ? 4 = 9

Practice 1

Work out the answers without a using a calculator.

1a. 21.5 + 8.75 =

1b. 15.6 ? 7.35 =

2a. A plane ticket costs ?20.99 plus ?12 airport taxes and ?13.50 for travel insurance. What is the total cost?

2b. A person weighs 72.4 kg. When they weigh themselves one month later, they weigh 67.7 kg. How much weight have they lost?

2c. A washing machine is on sale at ?295.49 at one store and ?288.99 at another. What is the difference in price?

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Application of Number Level 1

Decimal calculations

Multiply a decimal by a whole number

To multiply a decimal by a whole number, write down the decimal first and put the whole number underneath. Do the calculation in the same way as for whole numbers, then put the decimal point in your answer.

Count the number of decimal places in the number in the question. You need the same number of decimal places in the answer.

To check your answer, round to a whole number and do an approximate calculation.

Example 1: 5 ? 12.25 12.25

? 5 61.25

two decimal places (2 numbers after the point)

two decimal places (2 numbers after the point) Check: 5 ? 12 = 60

Example 2: 12 ? 7.24

You can either write this as a long multiplication (method 1), or break it into two parts (method 2: multiply by 10 and multiply by 2, then add the answers.

Method 1:

Method 2:

7.24

10 ? 7.24 = 72.4

? 12 7240 1448

2 ? 7.24 = 14.48 72.4 + 14.48 = 86.88

86.88

Check: 12 ? 7 = 84

Practice 2

Work out the answers without a using a calculator.

1a. 7 ? 3.45

1b. 12 ? 6.2

2a. A ticket for a concert costs ?15.70. How much will four tickets cost?

2b. A room measure 3 m by 4.6 m. What is the area in square metres?

2c. You pay household insurance in 12 monthly instalments of ?24.25. How much does it cost for the year?

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Application of Number Level 1

Decimal calculations

Dividing a decimal by a whole number

When you divide a decimal by a whole number, use the same method as for whole numbers. Put the decimal point in the answer above the decimal point in the number you are dividing.

To check your answer, round to a whole number and do an approximate calculation. You can also round your answer and multiply.

Example: 104.75 ? 5 2 0 . 9 5

) 5 1 0 4 . 7 5

Check: 105 ? 5 = 21 Check: 5 ? 21 = 105

Practice 3

Work out the answers without a using a calculator.

1a. 24.25 ? 5

b. 134.56 ? 4

2a. Four friends share a restaurant bill which comes to ?68.60. How much do they pay each?

2b. Three friends share a flat and share all the bills equally between them. A gas bill is ?70.56. How much do they pay each?

2c. A 5 kg bag of potatoes costs ?3.80. What is the cost per kilo?

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Application of Number Level 1

Decimal calculations

Check your skills

1 160.3 + 22.75 = A. 182.05 B. 183.5 C. 183.05 D. 387.8

2 A CD costs ?14.95 in the shops and ?10.99 online. How much cheaper is it online? A. ?4.04 B. ?4.40 C. ?4.06 D. ?3.96

3 Overtime is paid at ?8.15 per hour. How much does a worker get for five hours overtime? A. ?40 B. ?40.75 C. ?407.50 D. ?47.50

4 A room measures 3 m by 2.65 m. What is the area in square metres? A. 7.95 m2 B. 79.5 m2 C. 7.85 m2 D. 78.5 m2

5 A 5 litre tin of pain costs ?14.95. What is the cost per litre? A. ?2.19 B. ?2.99 C. ?2.91 D. ?29.90

Turn to page 6 to check your answers.

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Application of Number Level 1

Decimal calculations

Answers

Practice 1 1a. 30.25 2a. ?46.49

1b. 8.25 2b. 4.7kg

2c. ?6.50

Practice 2 1a. 24.15 2a. ?62.80

1b. 74.4 2b. 13.8 m2

2c. ?291

Practice 3 1a. 4.85 2a. ?17.15

1b. 33.64 2b. ?23.52

2c. ?0.76 = 76p

Check your skills

1 C 2 D 3 B 4 A 5 B

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Cymhwyso rhif Lefel 1

Cyfrifiadau degol

Cyflwyniad

Yn aml iawn, bydd cyfrifo arian neu fesurau metrig yn golygu defnyddio degolion. Fe allwch chi ddefnyddio cyfrifiannell, ond fydd gennych ddim cyfrifiannell wrth law bob amser - ac allwch chi ddim defnyddio cyfrifiannell mewn profion ac arholiadau - felly mae angen i chi wybod sut i gyfrifo ar bapur.

Mae'r gweithgaredd hwn edrych ar sut i gyfrifo gan ddefnyddio degolion. Byddwch yn dysgu sut i:

n adio a thynnu degolion n luosi degolyn ? rhif cyfan n rhannu degolyn ? rhif cyfan.

Bydd angen beiro a phapur arnoch chi i gyfrifo.

Hawlfraint ? Tribal Education 2007-2015

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Cymhwyso rhif Lefel 1

Cyfrifiadau degol Adio a thynnu degolion

Wrth adio a thynnu degolion, ysgrifennwch y rhifau mewn colofnau ac unionwch y pwyntiau degol. Sicrhewch fod pob digid yn y golofn iawn.

Rhowch y rhif `0' yn y colofnau gwag ar ?l y pwynt degol. I wirio'ch ateb, talgrynnwch eich atebion yn rhifau cyflawn a chyfrifo'r ateb yn fras.

Enghraifft 1: 2.75 + 15.9 + 6 2.75

+ 15.90 + 6.00

24.65

Gwiriwch: 3 + 16 + 6 = 25

Enghraifft 2: 12.9 ? 3.75 12.90 ? 3.75 9.15

Gwiriwch: 13 ? 4 = 9

Ymarfer 1

Cyfrifwch yr atebion heb ddefnyddio cyfrifiannell.

1a. 21.5 + 8.75 =

1b. 15.6 ? 7.35 =

2a. Mae tocyn hedfan yn costio ?20.99 ynghyd ? ?12 am drethi'r maes awyr a ?13.50 am yswiriant teithio. Beth yw cyfanswm y gost?

2b. Mae unigolyn yn pwyso 72.4kg. Wrth bwyso'i hun fis yn ddiweddarach, mae'n pwyso 67.7 kg. Faint o bwysau a gollodd yr unigolyn?

2c. Mae peiriant golchi dillad yn costio ?295.49 mewn un siop a ?288.99 mewn siop arall. Beth yw'r gwahaniaeth yn y prisiau?

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