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The speed of boat upstream and speed of boat downstream are 7 km/h and 13 km/h, respectively. Then the speed of stream and the speed of boat in still water is ?
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- 10 km/h and 3 km/h
- 15 km/h and 9 km/h
- 20 km/h and 6 km/h
- 40 km/h and 12 km/h