The velocity of a point moving along the $x$-axis is $v = 6 \sqrt { t } - 2 t$ units per second.

1. What was the average velocity over $0 ≤ t ≤ 15$ seconds?

2. During the first $15$ seconds, what is the total distance the point traveled?

3. What was the point's total displacement over $0 ≤ t ≤ 15$ seconds?

4. What accounts for the large difference between the answers to parts (b) and (c)?