#2**+1 **

The Fermi process is all about making rough estimates to get an order of magnitude type of result. So, for example:

Size of bathtub: ~ 2m x 1/2m *1/2m V_{bath} ~ 1/2m^{3} → 5*10^{5} cm^{3}

Size of brick: ~ 20cm x 10cm x 5cm V_{brick} ~ 10^{3} cm^{3}

Number of bricks in bath: V_{bath}/V_{brick} = 5*10^{5}/10^{3} = 500

Note: I simply estimated (i.e. guessed!) the sizes above; I didn't measure anything. Often a way of "estimating" is to choose a value that is clearly too small, and one that is clearly too big, then choose the geometric mean as the estimate (though I didn't do that above). This often works well if many people make an estimate of something and you take the geometric mean of the largest and smallest of their guesses!

Alan
Feb 22, 2018