Answer:
There are 11. Remember, there's a post at the very beginning of the fence, and one at the very end. So it's (fence length / 10) + 1 = (100 / 10) + 1 = 10 + 1 = 11.
They're positioned along the fence at the following distances from the start:
0 ft, 10 ft, 20 ft, ..., 100 ft.
That's probably why the question specified "straight-line fence". If it was in a decagon (10-sided polygon shape, so sort of round-ish), the first post could also serve as the last post, and there could be only 10 posts. But in a straight line, there have to be 11.
