A closed iron tank 12 m long, 9 m wide and 4 m deep is to be made.

Length (l) = 12 m

Breadth (b) = 9 m

Height (h) = 4 m

Total surface area of the tank = 2[lb + bh + hl]

= 2[12*9 + 9*4 + 12*4]

= 2[108 + 36 + 48]

$=384 \mathrm{~m}^{2}$

The Length of the Iron sheet $=\frac{\text { Area of the Iron Sheet }}{\text { Width of the Iron Sheet }}$

= 384/2

= 192 m.

Cost of the Iron Sheet = Length of the Iron Sheet * Cost rate

= 192 * 5

= Rs. 960