Discussion
A box opened at the top is made of wood of thickness 3 cm. Its external length, breadth and height are respectively 1.48 m, 1.16 m and 83 cm (base = length × breadth). What will be the cost of painting its inner surface at ₹ 150 per m2 ?
Answer: B
The cost is ₹ 839.10.
The wood is 3 cm thick on all four sides and the base, but the top is open, so the inner length and breadth lose 6 cm and the inner height loses only 3 cm: 142 cm, 110 cm and 80 cm.
\[ \begin{aligned} \text{base} &= 142 \times 110 = 15620\ \text{cm}^{2} \\ \text{walls} &= 2 \times 80 \times (142 + 110) = 40320 \\ \text{total} &= 55940\ \text{cm}^{2} = 5.594\ \text{m}^{2} \\ \text{cost} &= 5.594 \times 150 = 839.10 \end{aligned} \]
Subtracting 6 cm from the height as well, or painting a top that does not exist, gives one of the other figures.
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