B. 40
Feedback 20 buckets of water fill a tank when the capacity of each bucket is 18 litres.
∴ Capacity of the tank = 20 × 18 = 360 litres
∵ in the second case, Capacity of each bucket = 9 litres
Number of buckets needed = Total capacity of the tank/capacity of one bucket
∴ No. of buckets required = 360/ 9 = 40
Thus, 40 buckets are needed to fill the tank, if the capacity of each bucket is 9 litres.