12 buckets of water fill a tank when the capacity of each bucket is 13.5 litres. How many buckets will be needed to fill the same tank, if the capacity of each bucket is 9 litres?

Answer: Option D

Capacity of the tank
= (12 × 13.5) litres
= 162 litres
Capacity of each bucket = 9 litres
Number of buckets needed
$$\begin{array}{rl}& \text{= }\left(\frac{162}{9}\right)\\ & =18\end{array}$$