A boat moves relative to water with a velocity v and river is flowering with 2v. At what angle the boat shall move with the stream to have minimum drift?

For minimum drift, the boat's upstream velocity component relative to the ground must be zero, which means the angle theta between the boat's velocity and the stream direction satisfies v cos theta = 2v. Solving this equation gives cos theta = 1/2, so the angle theta is 120 degrees.

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