a person walks first at a constant speed of 4.90 m/s along a straight line from point a to point b and then back along the line from b to a at a constant speed of 2.90m/s.
(a) what is her average speed over the entire trip? (in m/s)

The average speed ever the entire trip was 3.9 m/s.

Explanation:

We can calculate the average speed of the person over the full trip as:

s = (4.9 m/s + 2.9 m/s) / 2 = 3.9 m/s

To calculate the average speed we don't need what is the total displacement, but only the speed in each section of the trip (4.9 m/s and 2.9 m/s) and the number of sections of the trip (2 sections).

conservation of mass

mass before something happens = mass afterwards