1.  Three pipes A, B and C can fill a tank in 6 hours. After working at it together for 2 hours, C is closed and A and B can fill the remaining part in 7 hours. The number of hours taken by C alone to fill the tank is: 

Answer: Option C Explanation:
C's 1 hour's work = { (A + B + C)'s 1 hour's work }  { (A + B)'s 1 hour's work }
C alone can fill the tank in 14 hours. 
2.  Three taps A, B and C can fill a tank in 12, 15 and 20 hours respectively. If A is open all the time and B and C are open for one hour each alternately, the tank will be full in: 

Answer: Option C Explanation:
Total time taken to fill the tank = (6 + 1) hrs = 7 hrs. 
3.  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? 

Answer: Option B Explanation: Time taken by one tap to fill half of the tank = 3 hrs.
So, total time taken = 3 hrs. 45 mins. 
4.  A large tanker can be filled by two pipes A and B in 60 minutes and 40 minutes respectively. How many minutes will it take to fill the tanker from empty state if B is used for half the time and A and B fill it together for the other half? 

Answer: Option D Explanation:
Suppose the tank is filled in x minutes.
x = 30 min. 
5.  One pipe can fill a tank three times as fast as another pipe. If together the two pipes can fill the tank in 36 minutes, then the slower pipe alone will be able to fill the tank in: 

Answer: Option C Explanation: Let the slower pipe alone fill the tank in x minutes.
x = 144 min. 
6.  Two pipes A and B can fill a tank in 15 minutes and 20 minutes respectively. Both the pipes are opened together but after 4 minutes, pipe A is turned off. What is the total time required to fill the tank? 

Answer: Option D Explanation:
The tank will be full in (4 min. + 10 min. + 40 sec.) = 14 min. 40 sec. 
7.  Two pipes A and B can fill a tank in 20 and 30 minutes respectively. If both the pipes are used together, then how long will it take to fill the tank? 

Answer: Option A Explanation:

8.  Two pipes A and B together can fill a cistern in 4 hours. Had they been opened separately, then B would have taken 6 hours more than A to fill the cistern. How much time will be taken by A to fill the cistern separately? 

Answer: Option C Explanation: Let the cistern be filled by pipe A alone in x hours. Then, pipe B will fill it in (x + 6) hours.
x^{2}  2x  24 = 0 (x 6)(x + 4) = 0 x = 6. [neglecting the negative value of x] 