If a body is traveling with constant speed , it means that it's distance is constantly increasing with time.
If it goes 5m in 3min , it will go 5m in next 3 min.
Average velocity is the total displacement divided by total time.
When the body travels 5m in 3 min and 25 m in next 3min , average velocity
=(25+5)/(3+3)
=30/6
=5m/min
