Answer
The two results are very different because in part (c), we found the average force of the continuous stream of pellets whereas in part (d), we found the average force specifically during the period of contact between each pellet and the wall.
Work Step by Step
In part (c), we found that the magnitude of the average force on the wall from the stream of pellets is $~~10~N$
In part (d), we found that the magnitude of the average force on the wall from each pellet is $~~1670~N$
The two results are very different because in part (c), we found the average force of the continuous stream of pellets whereas in part (d), we found the average force specifically during the period of contact between each pellet and the wall.
Note that the average force calculated in part (c) also includes all the time periods between each pellet strike when there was no contact, and thus no force, between the pellets and the wall.
