For the first part of the trip:
the car moves at 16 m/sec for 10 miles (which is 16093.44 meters)
velocity = distance / time
time1 = distance / velocity = 16093.44 / 16 = 1005.84 seconds
For the second part of the trip:
the car moves at 30 m/sec for another 10 miles (which is 16093.44 meters)
velocity = distance / time
time2 = distance / velocity = 16093.44 / 30 = 536.448 seconds
For the total trip:
total distance is 20 miles (which is 32186.88 meters)
total time = 1005.84 + 536.448 = 1542.288 seconds
average velocity = total distance / total time = 32186.88 / 1542.288
= 20.869 meters/second