How do I find the instantaneous velocity at time 0.55 s? Please help!

Question

anonymous · Answer

Time (s)             0.0 0.20 0.30 0.40 0.50 0.60 
Position (cm [W]) 0    25    75     75    75    0

anonymous · Answer

I tried this: 

Vin = VF- Vi 
         ------
          delta time

      = 37.5 cm [W} - 0
         --------------
                  5s
      =  7.5 cm/s [W]

anonymous · Answer

A good approximation of the instantaneous velocity at t=55s is the average velocity from t=50s to t=60s

anonymous · Answer

So.. what happens?

anonymous · Answer

because 50s to 60s if I plot the graph is 5 s..

anonymous · Answer

The average velocity from time t=50 to t=60 is:
v=(0-75)/(0.60-0.50)=-750cm/s. 
Because this is the average between the two times we assign it to the middle time t=55s. You can only estimate since you have discrete data

anonymous · Answer

ohh..

anonymous · Answer

'cause on the back of the book it says: 7.5 m/s [E]