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:

Let the thickness of the bottom be x cm
Then,
$${\mkern 1mu} {\left( {330 - 10} \right) \times \left( {260 - 10} \right) \times \left( {110 - x} \right)} = $$         $$8000 \times $$ $$1000$$
$$ \Rightarrow 320 \times 250 \times \left( {110 - x} \right) = 8000 \times 1000$$
$$\eqalign{ & \Rightarrow \left( {110 - x} \right) = \frac{{8000 \times 1000}}{{320 \times 250}} = 100 \cr & \Rightarrow x = 10\,{\text{cm}} = 1\,{\text{dm}} \cr} $$