Yes, they do:
MARTINI said if it takes you 80 min. to travel 60 km. then your AVERAGE speed is 45km/h. NO it is not AVERAGE speed. 45kmh is just the exact speed that you traveled the 60km and it took a duration of 80 min., as I said before. Also those numbers do not factor in both speeds, you think it does but look carefully because it doesn't. The reason why is because you have to do the formula twice (since there are two different speeds) to get the average.
EXAMPLE: If I traveled 30 km at 30kmh. Then traveled 30 km again at 90 kmh. What do you think my AVERAGE speed will be for the ENTIRE trip?????
30 kmh + 90 kmh = 120 kmh : 120 kmh / 2 (since two different speeds where used) = 60 kmh FOR THE ENTIRE DISTANCE of 60km.
These formulas Calculate SPEED, TIME and DISTANCE : SPEED = Distance / Time
TIME = Distance / Speed
DISTANCE = Speed x Time
They DO NOT calculate an AVERAGE.
http://www.onlinemat...d-problems.html (note the part that says "Be careful!" after the answer is given and then watch the video)