PSLE An open rectangular box is shown.
- The inside of the box including the base is painted. Find the painted area.
- When the box is packed full with 1-cm cubes, how many cubes are there?
- How many cubes touch inside the box as a result?
(a)
2 painted areas lengthwise
= 2 x 40 x 55
= 4400 cm2
2 painted areas breadthwise
= 2 x 35 x 55
= 3850 cm
2
Base painted area
= 40 x 35
= 1400 cm2
Painted areas
= 4400 + 3850 + 1400
= 9650 cm
2 (b)
Volume of 1 cube
= 1 x 1 x 1
= 1 cm
3 Volume of the box
= 40 x 35 x 55
= 77000 cm
3 Number of 1-cm cubes in the box when full
= 77000 ÷ 1
= 77000
(c)
Number of cubes that touch the base of the box
= 40 x 35
= 1400
Number of cubes that touch the 2 faces lengthwise excluding the base
= 2 x (55 - 1) x 40
= 2 x 54 x 40
= 4320
Number of cubes that touch the 2 faces breathwise excluding the base
= 2 x (35 - 2) x (55 - 1)
= 2 x 33 x 54
= 3564
Number of cubes that touch the inside of the box
= 1400 + 4320 + 3564
= 9284
Answer(s): (a) 9650 cm
2; (b) 77000; (c) 9284