16/20 = 80/100
multiply both top and bottom by 5 to go from 16/20 to 80/100
80/100 = 80% by definition of what "percent" means, which is "per 100" or "for every 100"
So 16/20 = 80% meaning 80% of the tomato is water
Note on a calculator, 16/20 = 0.80 which converts to 80% if you move the decimal point 2 spots to the right.