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

18

Total capacity of tank (capacity of each bucket is 13.5 liters) = 13.5 x 12 = 162 liters 

Buckets needed to fill the same tank with capacity of each bucket as 9 liters = \(\frac{162 }{9 }\) = 18 buckets