A wooden box (open at the top) of thickness 0.5 cm, length 21 cm, width 11 cm and height 6 cm is painted on the inside. The expenses of painting are Rs. 70. What is the rate of painting per square centimetres?
Explanation:
As it can be seen from the diagram, because of the thickness of the wall, the dimensions of the inside of the box is as follows: length = (21 – 0.5 – 0.5) = 20 cm, width
= (11 – 0.5 – 0.5) = 10 cm and height = (6 – 0.5) = 5.5 cm. Total number of faces to be painted = 4 walls + one base (as it is open from the top). The dimensions of two of the walls = (10 × 5.5), that of the remaining two walls = (20 × 5.5) and that of the base = (20 × 10). So the total area to be painted = 2 × (10 × 5.5) + 2 × (20 × 5.5) + (20 × 10) = 530 cm2. Since the total expense of painting this area is Rs. 70, the rate of painting = 70530 = 0.13 = Re 0.1 per sq.cm. (approximately).
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