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Three pipes A, B and C together can fill a cistern in 6 hours. After working together for 2 hours, C is closed and A and B fill the remaining cistern in 8 hours. Then the time in which the cistern can be filled by pipe C alone will be: 
A) 11 hours
B) 9 hours
C) 12 hours
D) 10 hours
Correct Answer : 12 hours Explanation : Three Pipes A,B and C together filled a tank in 6 hours A and B can fill remaining in 8 hours, (A + B)'s 8 hour's work = 2/3 Therefore C's 1 hour's work C alone can fill the tank in 12 hours 
Two pipes A and B can fill a cistern in 4 and 6 minutes respectively. If both the pipes kept turned on alternatively for one minute each starting from A, how long will it take to fill the cistern? 
A) 3(3/7) minutes
B) 6 minutes
C) 4(2/3) minutes
D) 6(6/7) minutes
Correct Answer : 4(2/3) minutes Explanation : As the pipes are operating alternatively for one minute, thus (A+B)'s 2 minutes job is =(1/4 +1/6) =5/12 Remaining part of cistern= 1  5/6 =1/6 Pipe A can fill 1/4 part of the cistern in 1 minute,and can fill 1/6 part of the cistern in = 4*(1/6) =2/3 minutes Total time taken to fill the Cistern = time taken to fill (5/6 part + 1/6 part) 
Pipe A can fill a tank in 45 hrs and pipe B can fill it in 36 hrs. If both the pipes are opened in the empty tank. In how many hours will it be full? 
A) 10 hr
B) 15 hr
C) 20 hr
D) 28 hr
Correct Answer : 20 hr Explanation : If a pipe can fill a tank in x hours, then : part filled in 1 hour = 1/x Part filled A in 1 hr= (1/45) 
If a pipe fills a tank in 6 h, then what part of the tank will the pipe fill in 1 h? 
A) 1/3
B) 1/6
C) 1/4
D) 1/5
Correct Answer : 1/6 Explanation : Let the pipe can fill a tank in x hours, then part filled in 1 hour = 1/x A pipe fills a tank in 6 h, then the part of tank filled in 1 h = 1/6 
A tap can fill a tank in 16 minutes and another can empty it in 8 minutes. If the tank is already half full and both the taps are opened together, the tank will be ? 
A) Filled in 12 min
B) Emptied in 12 min
C) Filled in 8 min
D) Emptied in 8 min
Correct Answer : Emptied in 8 min. Explanation : Let a pipe can fill a tank in x hours and another pipe can empty the full tank in y hours (where x > y), when opening both the pipes, the net part emptied in 1 hour = (1/y)(1/x) 
One tap can fill a cistern in 2 hours and another can empty the cistern in 3 hours. How long will they take to fill the cistern if both the taps are opened ? 
A) 5 hours
B) 6 hours
C) 7 hours
D) 8 hours
Correct Answer : 6 hours Explanation : Let a pipe can fill a tank in x hours, and another pipe can empty the full tank in y hours (where y> x), then on opening both the pipes, the net part filled in 1 hour = (1/x)(1/y) Hence, time taken to fill the cistern = 6 hours 

44 pipes can fill a large water tank in 27 hours. How many hours it take for 66 pipes to fill four such tanks ? 
A) 72
B) 54
C) 63
D) 84
Correct Answer : 72 Explanation : 44 pipes => 27 hours Amount of work done is same for filling a pipe, So 44*27 = 66*X 
A tank can be filled by one tap in 10 minutes and by another in 30 minutes. Both the taps are kept open for 5 minutes and then the first one is shut off. In how many minutes more is the tank completely filled? 
A) 5
B) 7.5
C) 10
D) 12 Correct Answer : 10 Explanation :
In 5 minutes work done by both pipes is = 5*(1/10 + 1/30) After 5 min first tap is closed, So, remaining work done by second tap is = 30*(1/3) = 10 