An emptying pipe A can empty the tank in 30 minutes It is opened in a tank which is full of water Af...........

Quantitative Aptitude ( CCS) Pipes&cistern

An emptying pipe A can empty the tank in 30 minutes. It is opened in a tank which is full of water. After 12 minutes another pipe B which can fill the tank in 15 minutes is also opened. In what total time will the tank get filled again?

8 minutes

12 minutes

15 minutes

24 minutes

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Explanation:

Explanation:
A can empty the tank in 30 minutes, it is opened for 12 minutes, so it emptied 1/30 * 12 = 2/5 of tank
Now 2/5 of this empty tank is to be filled by both the pipes
When both pipes are opened together, in 1 minute they fill = 1/15 – 1/30 = 1/30 of the tank
So 2/5 tank in 2/5 * 30 = 12 minutes
So total time = 12 + 12 = 24 minutes [earlier 12 minutes when A was opened, next 12 minutes when both opened]