A man travels 5km at the speed of 20kmph... from there he travels 20km at the speed of 40kmph.. what's the average speed?

Start by finding the time each part takes as this is a value you can add, unlike the speeds:
Time = Distance/Speed
5/20 = 0.25 hours = 15 minutes
20/40 = 0.5 = 30 minutes
Total distance = 25 km
Total time = 0.75 hours = 45 minutes
Speed = Distance/Time
Average speed = 25/0.75
Average speed = 33.3 kmph
If you need the answer in a different unit you will have to convert FIRST. Hope this was helpful.