Choice Ahoice B is the correct answer. To solve a subtraction equation, we add the same value to both sides.

1. Start with the equation: $x - 20 = 15$
2. Add 20 to both sides: $x = 15 + 20$
3. Calculate: $x = 35$
4. Verify: $35 - 20 = 15$ ✓

Addition consistently undoes subtraction, regardless of the specific numbers involved.

Choice Ahoice C is incorrect because this results from subtracting instead of adding: $15 - 20 = -5$. This applies subtraction twice.

Choice Ahoice D is incorrect because this might come from the absolute value of Choice Ahoice C, or from miscalculating $20 - 15 = 5$ (wrong order).

Choice Ahoice D is incorrect because this results from dividing: $\frac{15}{20} = 0.75$. Division is not relevant for solving subtraction equations.