When you illustrate the problem description, the direction of distance travelled is shown in the picture. But actually, since you are asked of the average velocity, you really don't need the drawing. It is given that the constant speed is 55 km/h. Since distance is speed multiplied with time, then you only have to multiply the constant speed and the respective time so you can find the distance. The average velocity, on the other hand, is the total distance per total time travelled. So since there is a constant speed, all the terms for time will be cancelled. Therefore, the average velocity is the constant speed. But we have to add the direction, which is south so it will denote a negative sign. The final answer for average velocity is -55 km/h.
