This is an English language forum - the sentences look fine.
If you want help on the maths, try here: http://www.mathhelpforum.com/math-help/
Two students are preparing for a marathon. Depinder ran 8 miles in 85 minutes. Kelly ran 6 miles in 55 minutes. Who has the faster average speed?
Do you know any Science forum, Anglika?
The best one you know.