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For part a, why do we use the current velocity and not the resultant velocity to solve how far downstream she will land?

Hi aheumangutman, "downstream" is the "x-direction" in the video. In order to know displacement in that direction (which is the "x-component" of the resultant displacement), we need to use the velocity along that direction. This is the "x-component" of the resultant velocity, but there's no need to do calculations for this "x-component" since the x-component is the speed of the current. In order to find the displacement along a certain direction, we need to know the velocity only along that direction.

Hope that helps,
Mr. Dychko