Tricky Mathematic Test

Woah ! Today mathematic test is 'quite' ! Not very easy not very difficult ! But i'm blank when i did the question just now ! Oh Geez, hope my score is better than before.

After the test, mamny of my friend are discussing the questions that have been tested. 1 of the question, TRICKED us ! and the qestion is :

" A boy go to school by cycling. He go to school with 30km/h and go home with 20km/h.What is the mean speed when he go and go home? " 

Many  of my friends answered 25km/h but when we ask the teacher he says 24km/h ! WTH? Geez. I was TOTALLY FALSE and i answered 25KM/H too !

Hope it was a bonus question. That's what we usually hope for when we get a test.