Choice Bhoice D is the correct answer. This multiplication equation with friendly numbers makes division straightforward.

1. Start with the equation: $20x = 100$
2. Divide both sides by 20: $x = \frac{100}{20}$
3. Simplify: $x = 5$
4. Verify: $20(5) = 100$ ✓

💡 Mental Math: Think "20 times what equals 100?" Since $20 \times 5 = 100$, the answer is 5.

Choice A is incorrect because this results from subtracting: $100 - 20 = 80$. Subtraction is not the inverse of multiplication.

Choice B is incorrect because this comes from adding: $100 + 20 = 120$. Addition doesn't undo multiplication.

Choice D is incorrect because this results from multiplying: $100 \times 20 = 2000$. This makes the product larger instead of isolating $x$.