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Diameter = 3m
radius = 3/2 = 1.5 m

volume= \frac{2}{3} \pi r^3 \\ \\ volume= \frac{2}{3} \times 3.14 \times 1.5^3 = 7.065\ m^3=7065l

emptying rate = 25/7 l per second
let the time taken to empty half volume = t

 \frac{25}{7} \times t= \frac{1}{2} \times volume\\ \\  t= \frac{1}{2} \times 7065 \times  \frac{7}{25} \\ \\   
t=\boxed{989\ seconds}