You cannot. There is technique that can be used to locate a fraction on a line segment. It involves the drawing of parallel lines passing through equidistant points drawn on an intersecting line. The procedure is based on Thales' theorem.
Draw a real line (passing through 0 of course) Mark the limits of segment [0,1]
From the point 0 draw a line. Choose an arbitrary length measured by the opening of a compass. Starting from O, mark three equal segments along the second line. From the end of the 3rd segment draw a line that joins that end with the end of the point 1 on the real line.
From each of the two other points on the secant draw segments parallel to the one you just drew.
Here is how it looks on a picture.
If you have another fraction (5/7) draw 7 equal length segments on the second line (secant to the real axis). The parallel line from the 5th point will cut the real axis at the point 5/7
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