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Pipes and Cistern - General Aptitude

Q1:

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:

A 10

B 12

C 14

D 16

ANS:C - 14

Part filled in 2 hours = 2 = 1
6 3
Remaining part = 1 - 1 = 2 .
3 3
(A + B)'s 7 hour's work = 2
3
(A + B)'s 1 hour's work = 2
21
C's 1 hour's work = { (A + B + C)'s 1 hour's work } - { (A + B)'s 1 hour's work }
   = 1 - 2 = 1
6 21 14
C alone can fill the tank in 14 hours.