An object travels North at #6 m/s# for #5 s# and then travels South at #7 m/s# for #3 s#. What are the object's average speed and velocity?
Average speed: 6.375 m/s
Average velocity: 1.125 m/s
Speed is a scalar quantity (not a vector), and therefore has no direction. Hence, you can just find the average speed by taking total distance travelled and dividing by total time travelled:
Velocity, however, is a vector, and therefore you'll need to take into consideration the direction you're running in.
You have to choose whether one of your directions to be the "positive" direction. I'll choose North as the positive direction, so we'll have:
The reason we subtract is because
If you chose south as the positive direction, then you'll get