Answer:
a). Average speed = 26 km per hour
b). Average velocity = 10 km per hour
Explanation:
a). Distance between A to B = 5 km
  Distance between B to C = 4 km
  Total distance traveled between A to B to C and then back to B
  = 5 + 4 + 4
  = 13 km
Time taken to cover this distance of 13 km = [tex]\frac{1}{2}[/tex] an hour
Therefore, average speed during this trip = [tex]\frac{13}{\frac{1}{2}}[/tex]
                                   = 26 km per hour
b). Average velocity = [tex]\frac{\text{Displacement}}{\text{Duration}}[/tex]
  Displacement = (Distance between A to B) + (Distance traveled between B to C) - (Distance traveled from C to B)
              = 5 + 4 - 4
              = 5 km towards A
Average velocity = [tex]\frac{5}{\frac{1}{2}}[/tex]
               = 10 km per hour
