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