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1. Pipe P can fill and empty tank in 4 hours but pipe Q can completely empty the same tank in 8 hours. But the pipes were opened alternately after every two hours starting with pipe P then in how many hours, the tank was completely filled ?
- A. 6 hours
- B. 6 hours
- C. 6 hours
- D. 6 hours
Answer: Option C
Explanation:
Let, Capacity of tank be 8 liter (LCM of 4,8)
In 1 hour P fill = (8/4) = 21
Q empty = (8/8) = 1L
In last 2 hour only p works and fill = 2x 2 = 4L
Remains = 8 - 4 = 4 L.
Now,
in 4 hour net filling of P and Q = (2x 2) - (1 x2) = 2L
2L net fill in 4 hr
4L net fill in = (4×4)/2 = 8 hr
Total time = 8 + 2 = 10 hr (Ans)
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