Quantitative Aptitude :: Pipes & Cistern - Discussion

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Explanation:
Tank part filled by pipe A in 1 minute = 1/60
Tank part filled by pipe B in 1 minute =1/40

So Tank Part filled by both pipes A and B in 1 minute = (1/60+1/40)=5/120=1/24

Lets Assume tanker get filled in x minutes.
Then, to fill the tanker from empty state, B is used for x/2 minutes
and A and B together is used for the rest x/2 minutes.

(x/2*1/40)+(x/2*1/24)=1
=> x/2(1/40+1/24)=1
=> x/2*8/120=1
=> x/30=1
=> x = 30 minutes