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Rowing is done always aiming in perpendicular direction to the flow of the river.
So net velocity of the boat has a velocity of 8 kmph perpendicular to the flow of river and 2 kmph along the flow of river water.

The boat crosses the river in  400 meters / 8 kmph = 1/20 Hours = 3 minutes

In 3 minutes, the boat will travel along the flow of the river
  = 1/20 hours * 2 kmph = 0.1 km

Hence the drift is 0.1 km.

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