Hi!
First of all we should calculate the displacement in the first 10 seconds : 3Ă—10=30m
And also we should calculate the displacement between the time 10s and 15s; which is |-6|Ă—5=30
Then the total displacement is 30+30=60
The totall time spent is 15s
So the speed is 60/15=4
But the answer of this question is B
Cause I didn’t pay attention to the velocity part🤦🏻‍♀️. Velocity is calculated by the the displacement divided by time. Displacement is related to the direction. But the speed is related to the direction and it’s calculated when you divide distance to time.
As this question is talking about velocity, the direction is important. So we have: deltaX=(30-30)/15=0
Cause the two displacements are in the different direction.