How do you find the average speed for a non constant speeds? 

Average speed is calculated by dividing the total distance travelled by the time interval. For example, someone who takes 40 minutes to drive 20 miles north and then 20 miles south (to end up at the same place), has an average speed of 40 miles divided by 40 minutes, or 1 mile per minute (60 mph).

The average speed is the distance travelled divide by the time taken: Average speed = Distance travelled time taken
Hoped this helped