Answer: 707 km/h
Step-by-step 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
33km total / 3 hours total = 11 km/hr average

Therefore x(x+1)≤0, and if two numbers multiply to a negative number, one of them has to be negative and the other has to be positive. In this case, because x+1>x, x≤0, and x+1≥0
Therefore -1≤x≤0, and x can take up the value of any real number in the interval [-1,0]
So the math here is 1+1 = 2