Choice Bhoice D is the correct answer.

1. Define: Distance = $d$. Time up + Time down = 3.
2. Equation: $\frac{d}{10} + \frac{d}{20} = 3$
3. Multiply by 20: $2d + d = 60$
4. Combine: $3d = 60$
5. Divide by 3: $d = 20$
6. Verify: $20/10 + 20/20 = 2 + 1 = 3$ ✓

Choice Bhoice A is incorrect because calculation error.

Choice Bhoice D is incorrect because calculation error.

Choice Bhoice C is incorrect because calculation error.