In the measurement of surface roughness using a 2D stylus profilometer, a surface profile measured over a length of 0.8 mm was recorded on a graph paper. During recording, vertical magnification of 10,000 and horizontal magnification of 100 were used. The areas in the as-recorded graph above and below the datum line are as follows:
Above (mm\(^2\)): 140, 60, 150, 50
Below (mm\(^2\)): 70, 50, 120, 160
The average surface roughness (\(R_a\)) of the surface is ______ \(\mu m\) (rounded off to two decimal places).
