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In a certain country 1/2 of 5 = 3. If the same proportion holds, what is the value of 1/3 of 10?

A while back I started a series of puzzles that seem difficult but have not-so-difficult answers. I can't find any of them, so I don't know how many exist. There must have been at least two, so I'll number this one 3. Have fun. A monotonic increasing function f (x) is cut above and below by horizontal lines that intersect it at f (x1) and f (x2). A vertical line is drawn through a point on the curve (black dot) between x1 and x2. The curve and the lines define the green and red areas shown on the figure. You are to find the point on the curve that minimizes the sum of these areas.