The train travels between the cities A and B which are 180km far away from each other, in 1 hour. During this travel the train keeps its speed constant for 4 minutes as 120km/h. During the last 6 minutes before arriving at the city B, the train decelerates and travels with an average speed of 100km/h. Excluding the time when the train travels with a constant speed and the time it decelerates, determine the average velocity of the train. Can you solve this step by step? In your entire response, the letter p should appear less than 29.
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