Ingrid is docking a motorboat. She turns off the power and lets the boat coast toward the dock. The distance, d, in metres, between the boat and the dock as a function of time, t, in seconds, is given by
d(t) = -7t+70/t+4 ,0 ≤ t ≤ 10.

a) What is the average velocity of the boat during the time interval from when Ingrid turns the boat off to when it meets the dock?