Answer: 707 km/h
Explanation:
let c = speed of current
16.5 + c = speed of sailboat downstream
16.5 - c = speed of sailboat upstream
distance = travel time * speed
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2(16.5 + c) = 5(16.5 - c)
33 + 2c = 82.5 - 5c
7c = 49.5
c = 7.07 km/h