The direction that an aircraft of given speed must fly in in order to intercept another aircraft of given velocity.
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If two particles are going to collide, then the relative velocity and position vectors of one of the particles are along the same line with the relative velocity vector pointing towards the other particle. This means that the vectors have opposite directions, so that one vector is a negative scalar multiple of the other vector.

In this problem, the relative direction (but not the relative speed) of a wind is given for two different velocities of an observer. From this we can find the (absolute) velocity of the wind.

3:50 The wind is actually blowing from the south east not the south west as shown here. However, by symmetry, we get the same results for the question.