Average speed is total distance travelled in total time. We can calculate average only when we are given any of these two.
In case, we are given only speed, then this formula is helpful.
V travelled half of his journey at X kmph and the remaining half at Y kmph. What is his average speed?
Average speed of V is = 2XY / X + Y.
Example: A travelled half of a distance is with 30Kmph and remaining half distance at 10Kmph. Then his average speed is:
2 × 30 × 10/30 + 10
= 600/40.
= 15Kmph.