Answer:
- boat: 70 km/h
- current: 21 km/h
Explanation:
Let b and c represent the speeds of the boat and current, respectively. Upstream, progress occurs at the rate that is the difference of these. Downstream progress is at the rate that is the sum of these.
speed = distance/time
b -c = 343/7 = 49
b +c = 819/9 = 91
Adding these equations gives ...
2b = 140
b = 70
Using this value in the second equation, we have ...
70 +c = 91
c = 21
The speed of the boat is 70 km/h; the speed of the current is 21 km/h.