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 of the thickness of the hair?

 
Magnificationm=Observed widthdReal widthx
Real widthx=3.5100mm
=0.035 mm