Answer:
6 km/h
Explanation:
After 4 hours at a speed difference of 9 km/h, the carrier made up a distance of ...
(4 h)(9 km/h) = 36 km
This head start was achieved by the container ship in 6 hours, so it speed was ...
(36 km)/(6 h) = 6 km/h
The container ship's average speed was 6 km/h.