A large cistern can be filled by two pipes P and Q in 15 minutes and 2...
Part filled by P and Q = 1/15 + 1/20 = 7/60
Part filled by Q = 1/20
x/2(7/60 + 1/20) = 12 minutes
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A large cistern can be filled by two pipes P and Q in 15 minutes and 2...
Part filled by P and Q = 1/15 + 1/20 = 7/60
Part filled by Q = 1/20
x/2(7/60 + 1/20) = 12 minutes
A large cistern can be filled by two pipes P and Q in 15 minutes and 2...
To solve this problem, we can use the concept of work. The work done by a pipe is inversely proportional to the time it takes to fill the cistern.
Let's assume that the capacity of the cistern is 1 unit.
Work done by pipe P in 1 minute = 1/15
Work done by pipe Q in 1 minute = 1/20
Work done by pipe Q in half the time = (1/20) * (1/2) = 1/40
Now, let's assume that it takes 'x' minutes to fill the cistern when Q is used for half the time and P and Q fill it together for the other half.
Work done by pipe P in x minutes = (1/15) * (x/2) = x/30
Work done by pipe Q in x minutes = (1/20) * (x/2) = x/40
The total work done by both pipes together is the sum of their individual work:
x/30 + x/40 = 1
To solve this equation, we need to find the least common multiple (LCM) of 30 and 40, which is 120. Multiplying both sides of the equation by 120, we get:
4x + 3x = 120
7x = 120
x = 120/7
Therefore, it will take approximately 17.14 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 we are looking for the closest whole number, the answer is 17 minutes, which corresponds to option A.