Arithmatical - Pipes and Cisterns

CATEGORY

DIRECTIONS : Problems based on Pipes&Cisterns.
16. Two pipes A and B can fill a tank in 6 hours and 4 hours respectively. If they are opened on alternate hours and if pipe A is opened first, in how many hours, the tank shall be full ?
  A.  4 hrs
  B.  5 hrs
  C.  7 hrs
  D.  9 hrs
Solution
A's work in 1 hour= 1/6.
B's work in 1 hour= 1/4.
(A+B)'s 2 hour's work when opened alternately= (1/6 + 1/4)
= 5/12.
(A+B)'s 4 hour's work when opened alternately= 10/12
= 5/6.
Remaining part= (1 - 5/6)
= 1/6.
Therefore, total tank to fill the tank(4+1)hrs = 5 hrs.
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