A box made of sheet metal costs ₹ 6480 at ₹ 120 per square metre.

Length of the box =5 m
Breadth of the box =3 m

Area of the sheet required $=\frac{\text { total cost }}{\text { cost per metre square }}$

Let h m be the height of the box.
Then area of the sheet = total surface area of the box    

$=2(l b+l h+b h) \mathrm{m}^{2}$

$=2(5 \times 3+5 \times h+3 \times h) \mathrm{m}^{2}$

$=2(15+8 h)=(30+16 h) \mathrm{m}^{2}$

Now, $30+16 h=\frac{6480}{120}$

$\Rightarrow 30+16 h=54$

$\Rightarrow 16 h=24$

$\Rightarrow h=1.5 \mathrm{~m}$

∴ The height of the box is 1.5 m.