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5.7 km/h north and 5.8 km/h west are instantaneous velocities, 8.1 km/h northwest is the average velocity.

Velocity is a vector quantity while speed is a scalar quantity. A vector quantity has both magnitude and direction. The direction of the vector must be taken into account in vector calculations.

We can see that at the two instants mentioned, the instantaneous velocities of the body were 5.7 km/h north and then went 5.8 km/h west. The average velocity must be obtained bearing in mind the direction of the two velocities hence 8.1 km/h northwest is the average velocity obtained from Pythagoras theorem as follows;

Vav= √(5.7)^2 + (5.8)^2 =  8.1 km/h

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