1. The problem asks to simplify the expression $(-10)(5x)(-8)$. 2. The rule for multiplication of numbers and variables is to multiply the coefficients (numbers) and keep the variable part as is. 3. Multiply the coefficients: $-10 \times 5 \times -8$. 4. Calculate step-by-step: - First, $-10 \times 5 = -50$. - Then, $-50 \times -8 = 400$ (since a negative times a negative is positive). 5. The variable $x$ remains as is, so the simplified expression is $400x$. 6. Therefore, the correct answer is B. $400x$.