A cistern of capacity 8000 litres measures externally 3.3 m by 2.6 m by 1.1 m and its walls are 5 cm thick. The thickness of the bottom is :

Answer: Option B

Let the thickness of the bottom be x cm
Then,
$$\left[\left(330-10\right)\times \left(260-10\right)\times \left(110-x\right)\right]$$       $$=8000\times 1000$$
$$\Rightarrow 320\times 250\times \left(110-x\right)=$$       $$8000\times 1000$$
$$\begin{array}{rl}& \Rightarrow \left(110-x\right)=\frac{8000\times 1000}{320\times 250}\\ & \Rightarrow \left(110-x\right)=100\\ & \Rightarrow x=10cm\\ & \Rightarrow x=1dm\end{array}$$