Let the length (l), breath (b), and height (h) be the external dimension of an open box and thickness be x.
The volume of metal used in box = Volume of external box – Volume of internal box
Consider external box,
Length, l = 36 cm
Breadth, b = 25 cm
Height, h = 16.5 cm
We know that the equation of the volume of cuboid is given by,
Volume of cuboid = lbh, where, l, b and h are the length, breadth and height of tank respectively
Volume of external box = 36(25)(16.5) = 14850 cm3
Since the box is open from top,
Consider internal box,
The thickness of two sides is reduced as follows,
Length, l’ = Length of external box – 2(thickness of box) = 36 – 2(1.5) = 33 cm
Breadth, b’ = Breadth of external box – 2(thickness of box) = 25 – 2(1.5) = 22 cm
Height, h’ = Height of external box – thickness of box = 16.5 – 1.5 = 15 cm
Volume of internal box = 33(22)(15) = 10890
And,
Volume of metal in box = 14850 – 10890 = 3960 cm3
We know that,
1 cm3 weighs 7.5 g
So, 3960 cm3 weighs 3960(7.5) = 29,700 g
Therefore, the weight of box is 29,700 g i.e. 29.7 kg