A boat takes $8 \text{ hours}$ to row $20 \text{ km}$ upstream and $30 \text{ km}$ downstream. It takes $11 \text{ hours}$ to row $28 \text{ km}$ upstream and $40 \text{ km}$ downstream. Find the speed of the boat in still water.

Options:

• A: $5 \text{ km/h}$
• B: $6 \text{ km/h}$
• C: $7 \text{ km/h}$
• D: $8 \text{ km/h}$

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