A water tank is 30 m long, 20 m wide and 12 m deep. It is made of iron sheet which is 3 m wide. The tank is open at the top. If the cost of the iron sheet is Rs. 10 per metre, then the total cost of the iron sheet required to build the tank is :

Since the tank is open at the top, we have :
Area of sheet required = Surface area of the tank
$$\eqalign{ & = lb + 2\left( {bh + lh} \right) \cr & = \left[ {30 \times 20 + 2\left( {20 \times 12 + 30 \times 12} \right)} \right]{{\text{m}}^2} \cr & = \left( {600 + 1200} \right){{\text{m}}^2} \cr & = 1800{\text{ }}{{\text{m}}^2} \cr} $$
Length of sheet required :
$$\eqalign{ & = \left( {\frac{{{\text{Area}}}}{{{\text{Width}}}}} \right) \cr & = \frac{{1800}}{3}m \cr & = 600\,m \cr} $$
∴ Cost of the sheet
= Rs. (600 × 10)
= Rs. 6000