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

A closed iron tank of dimensions $12 \mathrm{~m}$ long, $9 \mathrm{~m}$ wide and $4 \mathrm{~m}$ deep is to be made.

Surface area of the cuboidal tank $=2 \times($ length $\times$ breadth $+$ breadth $\times$ height $+$ length $\times$ height $)$

$=2 \times(12 \times 9+9 \times 4+12 \times 4)$

$=2 \times(108+36+48)$

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

Also, the cost of an iron sheet is Rs 5 per metre and the sheet is 2 metres wide.

i. e., area of a sheet $=1 \mathrm{~m} \times 2 \mathrm{~m}=2 \mathrm{~m}^{2}$

So, the cost of $2 \mathrm{~m}^{2}$ of iron sheet $=$ Rs 5

i.e., the cost of $1 \mathrm{~m}^{2}$ of iron sheet $=\mathrm{Rs} \frac{5}{2}$

$\therefore$ Cost of $384 \mathrm{~m}^{2}$ of iron sheet $=384 \times \frac{5}{2}=\mathrm{Rs} 960$