To express the commission structure as a piecewise function \( f(x) \), we need to consider both the commission rates and the ranges of sales amounts correctly.
The commission rates are:
- 3% for sales below $20,000
- 3.5% for sales from $20,000 to $30,000
- 4% for sales over $30,000
The correct piecewise function representing the commission \( f(x) \) is:
\[ f(x) = \begin{cases} 0.03x & \text{if } 0 \leq x < 20000 \ 0.035x & \text{if } 20000 \leq x \leq 30000 \ 0.04x & \text{if } x > 30000 \end{cases} \]
This function accounts for the actual commission amount by multiplying the sales amount by the respective commission rate for each range.
Thus, the correct selection from the provided options is:
\[ f(x) = \begin{cases} 0.03x & 0 \leq x < 20000 \ 0.035x & 20000 \leq x \leq 30000 \ 0.04x & x > 30000 \end{cases} \]