year 8 maths

294 Chapter 5 Ratios and rates
Example 11
Determining the scale factor
State the scale factor in the following situations.
a A length of 4 mm on a scale drawing represents an actual distance of 50 cm.
b An actual length of 0.1 mm is represented by 3 cm on a scaled drawing.
SOLUTION
a Scale ratio = 4 mm : 50 cm
= 4 mm : 500 mm
Scale ratio = 4 : 500
= 1 : 125
Scale factor = 125
b Scale ratio = 3 cm : 0.1 mm
= 30 mm : 0.1 mm
Scale ratio = 30 : 0.1
= 300 : 1
1
= 1:
300
Scale factor = 1
300
EXPLANATION
Write the ratio drawing length : actual length.
Convert to ‘like’ units.
Write the scale ratio without units.
Divide both numbers by 4 .
Ratio is now in the form 1 : scale factor.
The actual size is 125 times larger than the scaled
drawing.
Write the ratio drawing length : actual length.
Convert to ‘like’ units.
Write the scale ratio without units.
Multiply both numbers by 10 .
Divide both numbers by 300 .
Ratio is now in the form 1: scale factor .
The actual size is 300 times smaller than the scaled
drawing.
Exercise 5D
UNDERSTANDING AND FLUENCY
1–4, 5–6(½), 7 3, 4, 5–6(½), 7, 8(½)
5–8(½)
1 A map has a scale of 1 : 50 000 , therefore how many centimetres in real life does 1 cm on the map equal?
2 A map has a scale of 1 : 100 . How many metres on the map does 300 m in real life equal?
3 Convert 10 000 cm to:
a mm b m c km
4 Convert 560 m to:
a km b cm c mm
Example 9
5 Find the actual distance for each of the following scaled distances. Give your answer in an
appropriate unit.
a scale 1 : 10000 i 2 cm ii 4 mm iii 7.3 cm
b scale 1 : 20000 i 80 cm ii 18.5 mm iii 1.25 m
c scale 1 : 400 i 16 mm ii 72 cm iii 0.03 m
d s cale 1 : 300 i 5 mm ii 8.2 cm iii 7.1 m
e scale 1 : 2 i 44 m ii 310 cm iii 2.5 mm
f scale 1 : 5 i 4 m ii 24 cm iii 155 mm
g scale 1 : 0.5 i 12 cm ii 3.2 mm iii 400 m
h scale 100 : 1 i 3 cm ii 11.5 km iii 81.5 cm
Cambridge Maths NSW
Stage 4 Year 8 Second edition
ISBN 978-1-108-46627-1 © Palmer et al. 2018
Cambridge University Press
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