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primary 6 | Maths | Measurement
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Maple Chan
Maple Chan

primary 6 chevron_right Maths chevron_right Measurement chevron_right Singapore

Pls help! Unable to understand the answer key.

Date Posted: 4 years ago
Views: 463
snell
Snell
4 years ago
(a)
top+bottom = 2 x 9x4 = 72 cm²
left+right = 2 x 7x4 = 56 cm²
front+back = 2 x 9x7 = 126 cm²
total painted area
= 72+56+126 = 254 cm²

(b)
only the corner cubes have more than 1 face painted

how many corner cubes?

front&top row = 9x1 = 9
front&bottom row = 9x1 = 9

front&right row = (7-2)x1 = 5
front&left row = (7-2)x1 = 5
* -2 is to avoid double-count

back&top row = 9x1 = 9
back&bottom row = 9x1 = 9

back&right row = (7-2)x1 = 5
back&left row = (7-2)x1 = 5

left&top row = (4-2)x1 = 2
left&bottom row = (4-2)x1 = 2

right&top row = (4-2)x1 = 2
right&bottom row = (4-2)x1 = 2

total corner cubes = 9x4 + 5x4 + 2x4
= 64

total cubes in rectangular block
= 9x4x7 = 252

no. of cubes with 1 face painted
= 252 - 64
= 188


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Maple Chan
Maple Chan
4 years ago
Thank you v much but answer key is 118.

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AC Lim
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