Question 75
Two pipes can fill a tank in 45 and 30 minutes respectively. After some time the slower pipe was closed. Then the tank was filled by the other pipe in 13 minutes 20 seconds. When was the slower pipe closed after start?
Solution
Let say, slower pipe was closed after x min.
So, in x min they together filled
x(1/45+1/30)=x (2+3)/90=(x/18).
So, faster pipe will fill rest of it (1-x/18) or (18-x)/18 in (13+1/3) min.
In (13+1/3) min ,it fills=40/(45×3)=8/27.
So, (18-x)/18=8/27.
or, 18×27-27x=144.
or, x=12.67
So, A is correct choice.
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