Quantitative Aptitude :: Pipes & Cistern - Discussion

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Explanation:
For first 12 mins tank is 2/7 filled
For first 18 mins tank is 8/35 filled. i.e pipe A is opened for 18 mins and pipe B is opened for 6 mins. (3/7)-(1/5)=8/35. that means after 18 mins tank is 8/35 filled.
so, after 18 mins all the 3 pipes work simultaneously.
Now calculate the time required to fill the tank by 3 pipes,
(1/42)+(1/35)-(1/30)=2/105. It means 105/2 mins.
But the tank is already 8/35 filled.
The time taken to fill 27/35 of the tank=(105/2)*(27/35)= 40.5 mins.
The time taken to fill the tank= (18+40.5) mins = 58.5 mins.