1. The problem is to multiply two variables, $x_i$ and $f_i$. 2. When multiplying variables, you simply write them together as a product: $x_i \times f_i = x_i f_i$. 3. There are no special rules for multiplying indexed variables; treat them as any other variables. 4. So, the product of $x_i$ and $f_i$ is simply $x_i f_i$. Final answer: $$x_i f_i$$