20 buckets of water fill a tank when the capacity of each bucket is 14.5 litres. How many buckets will be required to fill the same tank if the capacity of each bucket is 10 litres?

One fill pipe A takes 3 minutes more to fill the cistern than two fill pipes A and B opened together to fill it. Second fill pipe B takes $$ {21{1\over 3}}$$ minutes more to fill cistern than two fill pipes A and B opened together to fill it. When will the cistern be full if both pipes are opened simultaneously?

Pipes P and Q can fill a tank in 18 and 27 minutes, respectively, whereas Pipe R can empty the full tank in 54 minutes.  P and Q were opened together for 6 minutes and then   closed and R was opened. The tank was emptied by R alone in :

A cistern can be filled by two pipes filling separately in 12 and 16 min respectively. Both pipes are opened together for a certain time but being obstructions, only 7/8 of full quantity water flows through the former and only 5/6 through the latter pipe. The obstructions, however, being suddenly removed, the cistern is filled in 3 minutes from that moment. How long was it obstructions before the full flow began? 

Pipes  A and B are emptying pipes and can empty a tank in 6 hours and 16 hours, respectively. C is a filling a pipe. All the three pipes were opened together. They took 80 minutes to empty $$ {5\over 8}th$$ of the tank. Pipe C alone can fill the tank in:

A tap can fill a tank in 6 hours. After half the tank is filled, three more similar taps are opened. What is the total time taken to fill the tank completely?

Four pipes P, Q, R and S can fill a cistern in 25, 20, 50 and 40 hours respectively. The pipe Q was opened at 7 pm, P at 9 pm, S at 10 pm and R at 11 pm. When will the cistern be full?

A tap can fill the tank in 8 hours and another can empty it in 16 hours. If both the taps are opened simultaneously, the time (in hours) to fill the tank is__.