An example - solution of the question „How many hairs are on a head? “
First, to get the area of the scalp we use a geometric approximation, which will be our model. We do this because the actual determination of that area would be difficult to make for any single head and because an exact result (say to 2-figure accuracy) would be representative of only that one head. Since heads vary in size even among adults (probably by 10% to 20%), the model will do as well for the approximation we’re trying to obtain. Now the model is a hemisphere of the same diameter as a typical adult head. That diameter is about 10 inches or 25 centimeters. Check your own head by look- ing in a mirror with a ruler across your face. The surface area of a hemisphere of radius r is half that of a spere.