A man covers $\frac{1}{3}$ of his journey at $10 \text{ km/h}$, the next $\frac{1}{3}$ at $20 \text{ km/h}$ and the remaining $\frac{1}{3}$ at $60 \text{ km/h}$. Find his average speed for the whole journey.

Options:

• A: $15 \text{ km/h}$
• B: $18 \text{ km/h}$
• C: $20 \text{ km/h}$
• D: $24 \text{ km/h}$

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