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A closed wooden box of thickness $0.5$ cm and length $21$ cm, width $11$ cm, and height $6$ cm, is panted on the inside. The cost of painting is Rs $70$. What is the rate of painting in rupees per sq. cm?

1. $0.7$
2. $0.5$
3. $0.1$
4. $0.2$

Since the box is to be painted from inside and thickness is 0.5 cm, dimension of the surface to be painted will be reduced by 0.5*2 = 1 cm.

So dimensions of inner box are, length = 20 cm, breadth = 10 cm and height = 5 cm

Total surface area = 2(lb + bh + lh) = 2(20*10 +10*5 + 20*5) = 700 sq. cm

$\therefore$ Rate of painting = 70/700 = Rs. 0.1 per sq. cm
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