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primary 6 | Maths
| Measurement
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Pls help! Unable to understand the answer key.
Date Posted: 5 years ago
Views: 488
(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
...
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
...
Thank you v much but answer key is 118.
See 1 Answer
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Hope this helps
Date Posted:
5 years ago
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