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Option 3 : 4xy/5(y - x)
Let the volume of the tank is V.
pipe A can fill the tank in x hours.
⇒ Rate at which pipe A fill the tank = V/x
Pipe B can empty the tank in y hours.
⇒ Rate at which Pipe B empty the tank = -(V/y)
Rate at which both the pipes can fill the tank = V(1/x - 1/y)
⇒ In 1 hr Pipe A and B can fill V(1/x - 1/y) volume of tank.
⇒ To fill 4/5 V, time taken to fill the tank = 4/5 V × (1/V(1/x - 1/y)) = 4xy/5(y - x)
∴ Both the pipes can fill the tank in (4xy/5(y - x))
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