Alright, so the average grain of sand is around 1/16 mm to 2mm. We'll say it's 2mm for this (you can plug in other numbers for 2 if your teacher says otherwise). In addition, the average diameter of a human cell is 100µm. 1000µm=1mm, so we can make it 2mm/100µm(since we want to see how many times 100um goes into 2mm)*1000µm/1mm (to cancel µm and mm out) to get 2*1000/100=2000/100=20