Two pipes A and B can fill a tank in 20 min and 40 min respectively A water pipe C can empty the tan...........

Quantitative Aptitude ( CCS) Pipes&cistern

Two pipes A and B can fill a tank in 20 min. and 40 min. respectively. A water pipe C can empty the tank in 20 min. First A and B are opened. After 7 min, C is also opened. In how much time, the tank is full?

30m

20m

25m

19m

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Explanation:

Answer & Explanation
Answer – 19m
Explanation :
Part filled in 7 min. = 7*((1/20)+(1/40))=7(2+1/40) = 21/40
Remaining part=(1-(21/40))=(19/40).
Net part filled in 1min. when A,B and C are opened=(1/20)+(1/40)-(1/20)=(1/40).
Now,(1/40) part is filled in one minute.
(19/40) part is filled in = (40*(19/40))=19minutes.