Why does the ball rolling westward appear to move east when observed from the outside of the truck

Because the truck is moving to the east faster than the ball is moving west.

Explanation:

In this problem the text reads:

Ball                                          Truck

Rolling west at 0.25 m/sMoving east at 1.0 m/s

You get asked if the observer in the truck would see the ball moving to the west, the answer is yes, cause the observer in the truck would be moving to the east along with the truck, but someone looking at the ball from the outside will see the ball moving east because the speed of the ball is less than that of the truck.

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the part of the wave he is travelling on is the crest