Question 103082
We are told d = distance traveled = 21 km. 
We are told average speed = 36 km/hr.
The standard distance equation is: d = rt. We know r=36 and d=21, but we don't know t, so we find it by dividing d by r:
21/36=t
We can simplify 21/36 to be 7/12 of an hr.

We know an hour is 60 minutes and that 5*12=60, so 35 minutes must be the total time allowed to cover 21km to have an average speed of 36km/hr.

This is a good place to check your intermediate result.
Does rt = d? That is does 36*(35/60) = 21? Yes, it does.

We also are told the train is in motion for only 20 minutes. That means it is sitting in the station for 15 minutes.

We know there are 6 stops, but would we keep the timer running for the at the last stop. That is, would we stop the timer when we got to the last station or not? I assume we would. But I do not think we would count the station where we got onto the train as a stop...it was the start! (smile)

So, there actually were 5 stops along the way that took time. 15/5 = 3 minutes per stop (max).