To determine what type of circles should be used for the endpoints of the function \( f(x) = 0.3x \) in the specified range \( 0 \leq x \leq 20,000 \):
- The function \( f(x) = 0.3x \) is defined for \( x \) values inclusive of both endpoints, as indicated by \( 0 \leq x \leq 20,000 \).
- This means that at \( x = 0 \) and \( x = 20,000 \), the values of the function are included in the function's output.
Since both endpoints are included in the range, we should use closed circles at both endpoints when graphing this part of the function.
Therefore, the correct response is: Both ends should have closed circles.