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,
$$\left(330-10\right)\times \left(260-10\right)\times \left(110-x\right)=$$         $$8000\times$$ $$1000$$
$$\Rightarrow 320\times 250\times \left(110-x\right)=8000\times 1000$$
$$\begin{array}{rl}& \Rightarrow \left(110-x\right)=\frac{8000\times 1000}{320\times 250}=100\\ & \Rightarrow x=10\text{cm}=1\text{dm}\end{array}$$