A student measures the thickness of a human hair

A student measures the thickness of a human hair by looking at it through a microscope of magnification 100. He makes 20 observations and finds that the average width of the hair in the field of view of the microscope is 3.5 mm. What is the estimate on the thickness of hair?

Magnification of microscope = 100
Observed width of the hair = 3.5 mm
Estimate on the thickness of hair is given by
Magnification =Observed width / Real width or
Real width = Observed width / Magnification = 3.5 /100
= 0.035 mm