A plane flies 600 kilometers away from its bad at 400 meters per second then flies back to its base at 600 meters per second. What is it's average velocity?

Seems to me that it flies 400 m/s there and 600 m/s back the same distance.

therefore the average of 400 and 600 is 500 m/s. The distance is the same so the normal formula of (d2-d1) / (t2-t1) is applicable.