1)Draw a line of 8.1 cmname it AB.OR
2)From B again draw line of 1 cm name it C OR
3)find the center of A and C through perpendicular bisector.
4)name the center as O.With O as center and OA or OC as radius draw a semicircle.(such that one end of the circle should be on A and other on C)
5)Now draw 90 degree on B.such that it should intersect the semicircle at point D.
7)With B as center and BD as radius draw an arc intersecting the number line.name the point as E.
8)E represents route 8.1 on the number line