A large cistern can be filled by two pipes P and Q in 15 minutes and 1...
Part filled by P and Q = 1/15 + 1/10 = 1/6
Part filled by Q = 1/10
x/2(1/6 + 1/10) = 2/15 = 15/2 = 7.5 minutes
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A large cistern can be filled by two pipes P and Q in 15 minutes and 1...
To solve this problem, we can use the concept of work. The work done by a pipe is equal to the product of its rate and the time it takes to fill the cistern.
Let's assume that the capacity of the cistern is 1 unit (you can assume any value as it will not affect the final answer).
The rate of pipe P is 1/15 units per minute, and the rate of pipe Q is 1/10 units per minute.
Let's break down the problem into two parts:
1. Q is used for half the time:
Since Q is used for half the time, it will fill the cistern for 1/2 * t minutes, where t is the total time taken to fill the cistern.
The work done by Q in 1/2 * t minutes is (1/2 * t) * (1/10) = t/20 units.
2. P and Q fill it together for the other half:
Since P and Q fill the cistern together for the other half of the time, they will fill the cistern for 1/2 * t minutes.
The combined rate of P and Q is (1/15) + (1/10) = 5/30 + 3/30 = 8/30 units per minute.
The work done by P and Q together in 1/2 * t minutes is (1/2 * t) * (8/30) = 4t/60 = t/15 units.
Now, we can set up an equation to find the total work done:
t/20 + t/15 = 1
Multiplying through by 60 to clear the fractions, we get:
3t + 4t = 60
7t = 60
t = 60/7
So, it will take approximately 8.57 minutes to fill the cistern from an empty state if Q is used for half the time and P and Q fill it together for the other half.
Since the options given are in minutes, we round off the answer to the nearest minute, which is 9 minutes.
Therefore, the correct answer is option B) 7.5 minutes.