SOLUTION: Pipe A can fill a tank in 12 hours and pipe B can empty the tank in 18 hours.Both pipes are opened at 6 am & after some time pipe B is closed,&the tank is full at 8 pm on the same

Question 825011: Pipe A can fill a tank in 12 hours and pipe B can empty the tank in 18 hours.Both pipes are opened at 6 am & after some time pipe B is closed,&the tank is full at 8 pm on the same day.At what time was the pipe B closed?
A)8 am
B)9 am
Answer by josgarithmetic(39625) (Show Source): You can put this solution on YOUR website!
The time between 6am and 8pm is made of two time periods, x and y. One combination of pipes was used for x amount of time, and another combination of pipes was used for y amount of time.

Sum of the time periods was 14 hours. That is the amount of time from 6am to 8pm. We have our very important first equation:

The filling rate when both pipes A and B are open is tanks per hour)

When both tanks were open for x hours, the amount of tank filled was (1/36)x.
When only tank A was open for y hours, the amount of tank DURING y was (1/12)y.
The whole tank filled was the SUM of these amounts of the filling job:

The equations highlight-enclosed in red are your system to solve. You might want to simplify the job sum equation first, multiplying left and right by 36:
,
'
So now you can use the simpler system to solve for x and y:
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