Speed and acceleration
Speed is distance divided by time and acceleration is change in speed divided by time, so keep the two rates apart.
A worked example
A train covering 210 km in 2.5 h averages 210 / 2.5 = 84 km/h, and a car reaching 24 m/s from rest in 6 s accelerates at 24 / 6 = 4 m/s2.
The mistake people actually make
Average speed uses total distance over total time, including any time spent stopped, so it is not the average of the separate speeds. Convert minutes to hours before dividing if the answer is wanted in km/h.