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1. Two pipes A & B would be fill a tank in 20 & 30 minutes respectively. Both pipes being oppen, find when A must be turned off so that the tank may be just filled in 15 minutes.

  • A. After 10 minutes
  • B. After 10 minutes
  • C. After 10 minutes
  • D. After 10 minutes

Answer: Option A

Explanation:

Let the time be T minutes after which pipe A need to turned off. Both of the pipes remain open together only for T minutes.

Pipe A fills 1/20 of the tank in 1 minute.

Pipe B fills 1/30 of the tank in 1 minute

Together in 1 minute they fill (1/20 + 1/30 = 1/12) of the tank.

Together in T minutes they fill = T/12 of the tank

In (15 - T) minutes, pipe B alone can fill = (15 - T)/30 of the tank

Now, T/12 +  (15 - T)/30 = 1

or,  {15T + 6(15 - T)}/180 = 1

or, (15T + 90 - 6T) / 180 = 1

or,  9T = 180 - 90

or,  9T = 90

So, T =  10 minutes

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