PSLE
Justin had a rectangular block of wood 18 cm by 13 cm by 10 cm. He painted all the faces of the block.

1. What is the total painted area?
2. Justin then cut the block into 1-cm cubes. How many of these cubes have none of the faces painted?
3. How many of these cubes have 2 of the faces painted?

(a)
Total surface area of the top and the bottom
= 18 x 13 x 2
= 468 cm²

Total surface area of the front and the back
= 18 x 10 x 2
= 360 cm²

Total surface area of the left and right
= 13 x 10 x 2
= 260 cm²

Total painted area
= 468 + 360 + 260
= 1088 cm²

(b)
Dimension of the cuboid that does not have any painted surface.

Length
= 18 - 1 - 1
= 16 cm

Breadth
= 13 - 1 - 1
= 10 cm

Height
= 10 - 1 -1
= 8 cm

Number of cubes without any painted surface
= ^{16 x 10 x 8}_{1 x 1 x 1}
= 1280

(c)
Number of cubes with 2 faces painted lengthwise
= 16 x 4
= 64

Number of cubes with 2 faces painted breadthwise
= 10 x 4
= 40

Number of cubes with 2 cubes painted heightwise
= 8 x 4
= 32

Total number of cubes that have 2 painted surfaces
= 64 + 40 + 32
= 136

Answer(s): (a) 1088 cm²; (b) 1280; (c) 136